Can toilet or sewage aerosol transfer eggs of human parasites?

Can toilet or sewage aerosol transfer eggs of human parasites?

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Flushing a toilet produces hundreds of thousands of tiny droplets containing viruses and bacteria (source).

Additionally vomit and feces of infected people sometimes contain extremely high concentrations of viruses or bacteria.

Similarly, sewage water produces droplets and was proposed as the transmission mode in the SARS outbreak in Hong Kong in 2003.

Apart from viruses and bacteria being transferred through toilet and sewage aerosol, can human parasite eggs be transferred as well given the droplet size?

I'm interested in the aerosol, not the relatively large droplets that land on the toilet itself.

Common small protozoa that are present in infective loads in stool have not been documented in toilet aerosols. The size of the organism is going to be an issue here. As you can see from this table from the CDC, intestinal parasites (and ova) of concern to human health are larger than most droplets. Compare, E. coli and Shigella (0.5 - ~2 $mu m$). Salmonella tends to be a litter larger (up to 5$mu m$), but I expect those 5 $mu m$ specimens are not the ones transmitted through aerosols. Cryptosporidium would probably be the most likely candidate, and that's one shown in the first linked study to not be present in aerosols. I wouldn't say it's impossible, but certainly transmission through aerosolized particles is less likely than for the smaller viral and bacterial organisms.

General Information

Giardiasis is a diarrheal disease caused by the microscopic parasite Giardia duodenalis (or &ldquoGiardia&rdquo for short). Once a person or animal has been infected with Giardia, the parasite lives in the intestines and is passed in stool (poop). Once outside the body, Giardia can sometimes survive for weeks or even months. Giardia can be found in every region of the United States and around the world.

How do you get giardiasis and how is it spread?

You can get giardiasis if you swallow the Giardia parasite (germ). Giardia&mdashor poop from people or animals infected with Giardia&mdashcan contaminate anything it touches. Giardia spreads very easily even getting tiny amounts of poop in your mouth could make you sick.

Giardiasis can be spread by:

  • Swallowing unsafe food or water contaminated with Giardia germs
  • Having close contact with someone who has giardiasis, particularly in childcare settings
  • Traveling within areas that have poor sanitation
  • Exposure to poop through sexual contact from someone who is sick or recently sick with Giardia
  • Transferring Giardia germs picked up from contaminated surfaces (such as bathroom handles, changing tables, diaper pails, or toys) into your mouth
  • Having contact with infected animals or animal environments contaminated with poop

What are the symptoms of giardiasis?

Giardia infection (giardiasis) can cause a variety of intestinal symptoms, which include:

  • Diarrhea
  • Gas
  • Foul-smelling, greasy poop that can float
  • Stomach cramps or pain
  • Upset stomach or nausea
  • Dehydration

Symptoms of giardiasis generally begin by having 2 to 5 loose stools (poop) per day and progressively increasing fatigue. Other, less common symptoms include fever, itchy skin, hives, and swelling of the eyes and joints. Over time, giardiasis can also cause weight loss and keep the body from absorbing nutrients it needs, like fat, lactose, vitamin A, and vitamin B12. Some people with Giardia infections have no symptoms at all.

How long after infection do symptoms appear?

Symptoms of giardiasis normally begin 1 to 2 weeks after becoming infected.

How long will symptoms last?

Symptoms generally last anywhere from 2 to 6 weeks. In people with weakened immune systems (e.g., due to illness such as HIV), symptoms may last longer. Healthcare providers can prescribe the appropriate antiparasitic medications to help reduce the amount of time symptoms last.

Who is most at risk of getting giardiasis?

Anyone can become infected with Giardia. However, those at greatest risk are:

  • People in childcare settings
  • People who are in close contact with someone who has the disease
  • Travelers within areas that have poor sanitation
  • People who have contact with poop during sexual activity
  • Backpackers or campers who drink untreated water from springs, lakes, or rivers
  • Swimmers who swallow water from swimming pools, hot tubs, splash pads, or untreated recreational water from springs, lakes, or rivers
  • People who get their household water from a shallow well
  • People with weakened immune systems
  • People who have contact with infected animals or animal environments contaminated with poop

How is giardiasis diagnosed?

Contact your healthcare provider if you think you may have giardiasis. Your healthcare provider will ask you to submit stool (poop) samples to see if you are infected. Because it can be difficult to detect Giardia, you may be asked to submit several stool specimens collected over several days to see if you are infected.

What is the treatment for giardiasis?

Many prescription drugs are available to treat giardiasis. Although Giardia can infect all people, infants and pregnant women may be more likely to experience dehydration from the diarrhea caused by giardiasis. To prevent dehydration, infants and pregnant women should drink a lot of fluids while sick. Dehydration can be life-threatening for infants, so it is especially important that parents talk to their healthcare providers about treatment options for their infants.

My child does not have diarrhea but was recently diagnosed with giardiasis. My healthcare provider says treatment is not necessary. Is this correct?

Your child may not need treatment if they have no symptoms, though it is important to consider that their poop may remain a source of infection for other household members for an uncertain period of time. However, if your child does not have diarrhea but does have other symptoms, such as nausea or upset stomach, tiredness, weight loss, or a lack of hunger, you and your healthcare provider may need to consider treatment. The same is true if many family members are sick or if a family member is pregnant and unable to take the most effective medications to treat Giardia. Contact your healthcare provider for specific treatment recommendations.

Can I get giardiasis from my private well?

Giardia-contaminated poop can enter ground water through different ways, including sewage overflows, sewage systems that are not working properly, and polluted storm water. Wells may be more likely to be contaminated by poop after flooding, particularly if the wells are shallow, have been dug or bored, or have been covered by floodwater for long periods of time. Overused, leaky, or poorly maintained septic systems could contaminate nearby wells with germs from poop, including Giardia. Read more about testing your well.

What can I do to prevent and control giardiasis?

To prevent and control Giardia infection, it is important to:

  • Wash your hands with soap and water during key times, especially:
    • before preparing food or eating, and
    • after using the bathroom or changing diapers.
      from springs, lakes, or rivers (surface water) while backpacking or camping if no other source of safe water is available.
  • Avoid swallowing water from swimming pools, hot tubs, splash pads, and untreated water from springs, lakes, or rivers (surface water) while swimming.
  • Store, clean, and prepare fruits and vegetables properly.
  • Can I get giardiasis from my pet?

    The chances of people getting a Giardia infection from dogs or cats are small. The type of Giardia that infects humans is usually not the same type that infects dogs and cats. For more information, see Giardia and Pets.

    Toxoplasmosis: General FAQs

    Toxoplasmosis is an infection caused by a single-celled parasite called Toxoplasma gondii. While the parasite is found throughout the world, more than 40 million people in the United States may be infected with the Toxoplasma parasite. The Toxoplasma parasite can persist for long periods of time in the bodies of humans (and other animals), possibly even for a lifetime. Of those who are infected however, very few have symptoms because a healthy person&rsquos immune system usually keeps the parasite from causing illness. However, pregnant women and individuals who have compromised immune systems should be cautious for them, a Toxoplasma infection could cause serious health problems.

    How do people get toxoplasmosis?

    A Toxoplasma infection occurs by one of the following:

    • Eating undercooked, contaminated meat (especially pork, lamb, and venison) or shellfish (for example, oysters, clams or mussels).
    • Accidental ingestion of undercooked, contaminated meat or shellfish after handling them and not washing hands thoroughly (Toxoplasma cannot be absorbed through intact skin).
    • Eating food that was contaminated by knives, utensils, cutting boards and other foods that have had contact with raw, contaminated meat or shellfish.
    • Drinking water contaminated with Toxoplasma gondii.
    • Accidentally swallowing the parasite through contact with cat feces that contain Toxoplasma. This might happen by
      • Cleaning a cat&rsquos litter box when the cat has shed Toxoplasma in its feces
      • Touching or ingesting anything that has come into contact with cat feces that contain Toxoplasma or
      • Accidentally ingesting contaminated soil (e.g., not washing hands after gardening or eating unwashed fruits or vegetables from a garden).
      • Mother-to-child (congenital) transmission.
      • Receiving an infected organ transplant or infected blood via transfusion, though this is rare.

      What are the signs and symptoms of toxoplasmosis?

      Symptoms of the infection vary.

      • Most people who become infected with Toxoplasma gondii are not aware of it because they have no symptoms at all.
      • Some people who have toxoplasmosis may feel as if they have the &ldquoflu&rdquo with swollen lymph glands or muscle aches and pains that may last for a month or more.
      • Severe toxoplasmosis, causing damage to the brain, eyes, or other organs, can develop from an acute Toxoplasma infection or one that had occurred earlier in life and is now reactivated. Severe toxoplasmosis is more likely in individuals who have weak immune systems, though occasionally, even persons with healthy immune systems may experience eye damage from toxoplasmosis.
      • Signs and symptoms of ocular toxoplasmosis can include reduced vision, blurred vision, pain (often with bright light), redness of the eye, and sometimes tearing. Ophthalmologists sometimes prescribe medicine to treat active disease. Whether or not medication is recommended depends on the size of the eye lesion, the location, and the characteristics of the lesion (acute active, versus chronic not progressing). An ophthalmologist will provide the best care for ocular toxoplasmosis.
      • Most infants who are infected while still in the womb have no symptoms at birth, but they may develop symptoms later in life. A small percentage of infected newborns have serious eye or brain damage at birth.

      Who is at risk for developing severe toxoplasmosis?

      People who are most likely to develop severe toxoplasmosis include:

      • Infants born to mothers who are newly infected with Toxoplasma gondii during or just before pregnancy.
      • Persons with severely weakened immune systems, such as individuals with AIDS, those taking certain types of chemotherapy, and those who have recently received an organ transplant.

      What should I do if I think I am at risk for severe toxoplasmosis?

      If you are planning to become pregnant, your health care provider may test you for Toxoplasma gondii. If the test is positive it means you have already been infected sometime in your life. There usually is little need to worry about passing the infection to your baby. If the test is negative, take necessary precautions to avoid infection (See below).

      If you are already pregnant, you and your health care provider should discuss your risk for toxoplasmosis. Your health care provider may order a blood sample for testing.

      If you have a weakened immune system, ask your doctor about having your blood tested for Toxoplasma. If your test is positive, your doctor can tell you if and when you need to take medicine to prevent the infection from reactivating. If your test is negative, it means you need to take precautions to avoid infection. (See below).

      What should I do if I think I may have toxoplasmosis?

      If you suspect that you may have toxoplasmosis, talk to your health care provider. Your provider may order one or more varieties of blood tests specific for toxoplasmosis. The results from the different tests can help your provider determine if you have a Toxoplasma gondii infection and whether it is a recent (acute) infection.

      What is the treatment for toxoplasmosis?

      Once a diagnosis of toxoplasmosis is confirmed, you and your health care provider can discuss whether treatment is necessary. In an otherwise healthy person who is not pregnant, treatment usually is not needed. If symptoms occur, they typically go away within a few weeks to months. For pregnant women or persons who have weakened immune systems, medications are available to treat toxoplasmosis.

      How can I prevent toxoplasmosis?

      There are several steps you can take to reduce your chances of becoming infected with Toxoplasma gondii.

      • Cook food to safe temperatures. A food thermometer should be used to measure the internal temperature of cooked meat. Color is not a reliable indicator that meat has been cooked to a temperature high enough to kill harmful pathogens like Toxoplasma. Do not sample meat until it is cooked. USDA recommends the following for meat preparation:
        • For WholeCuts of Meat (excluding poultry)
          Cook to at least 145° F (63° C) as measured with a food thermometer placed in the thickest part of the meat, then allow the meat to rest* for three minutes before carving or consuming. *According to USDA, &ldquoA &lsquorest time&rsquo is the amount of time the product remains at the final temperature, after it has been removed from a grill, oven, or other heat source. During the three minutes after meat is removed from the heat source, its temperature remains constant or continues to rise, which destroys pathogens.&rdquo
        • For Ground Meat (excluding poultry)
          Cook to at least 160° F (71° C) ground meats do not require a rest time.
        • For All Poultry (whole cuts and ground)
          Cook to at least 165° F (74° C) . The internal temperature should be checked in the innermost part of the thigh, innermost part of the wing, and the thickest part of the breast. Poultry do not require a rest time.
        • Freeze meat* for several days at sub-zero (0° F) temperatures before cooking to greatly reduce chance of infection. *Freezing does not reliably kill other parasites that may be found in meat (like certain species of Trichinella) or harmful bacteria. Cooking meat to USDA recommended internal temperatures is the safest method to destroy all parasites and other pathogens.
        • Peel or wash fruits and vegetables thoroughly before eating.
        • Do not eat raw or undercooked oysters, mussels, or clams (these may be contaminated with Toxoplasma that has washed into seawater).
        • Do not drink unpasteurized goat&rsquos milk.
        • Wash cutting boards, dishes, counters, utensils, and hands with soapy water after contact with raw meat, poultry, seafood, or unwashed fruits or vegetables.
        • Wear gloves when gardening and during any contact with soil or sand because it might be contaminated with cat feces that contain Toxoplasma. Wash hands with soap and water after gardening or contact with soil or sand.
        • Ensure that the cat litter box is changed daily. The Toxoplasma parasite does not become infectious until 1 to 5 days after it is shed in a cat&rsquos feces.
        • Wash hands with soap and water after cleaning out a cat&rsquos litter box.
        • Teach children the importance of washing hands to prevent infection.

        If you have a weakened immune system, please see guidelines for Immunocompromised Persons. For further information on safe food handling to help reduce foodborne illness visit the Fight BAC! ® website External external icon .

        If I am at risk, can I keep my cat?

        Yes, you may keep your cat if you are a person at risk for a severe infection (e.g., you have a weakened immune system or are pregnant) however, there are several safety precautions you should take to avoid being exposed to Toxoplasma gondii, including the following:

        • Ensure the cat litter box is changed daily. The Toxoplasma parasite does not become infectious until 1 to 5 days after it is shed in a cat&rsquos feces.
        • If you are pregnant or immunocompromised:
          • Avoid changing cat litter if possible. If no one else can perform the task, wear disposable gloves and wash your hands with soap and water afterwards.
          • Keep cats indoors. This is because cats become infected with Toxoplasma through hunting and eating rodents, birds, or other small animals that are infected with the parasite.
          • Do not adopt or handle stray cats, especially kittens. Do not get a new cat while you are pregnant or immunocompromised.
          • Feed cats only canned or dried commercial food or well-cooked table food, not raw or undercooked meats.
          • Keep your outdoor sandboxes covered.

          Your veterinarian can answer any other questions you may have regarding your cat and risk for toxoplasmosis.

          Once infected with Toxoplasma is my cat always able to spread the infection to me?

          No, cats only spread Toxoplasma in their feces for 1-3 weeks following infection with the parasite. Like humans, cats rarely have symptoms when infected, so most people do not know if their cat has been infected. Your veterinarian can answer any other questions you may have regarding your cat and risk for toxoplasmosis.

          5.3 Methods on how some important parasites are spread


          Giardia occur in the intestines of humans. When Giardia are inside the body they can move about quite easily, but they often leave the body as tiny egg-like cysts in faeces.

          Infection happens when these cysts are taken back into the body of someone who does not have Giardia in their intestines. Once inside the intestine they become mobile (able to move) again and start to reproduce themselves by dividing and redividing.

          • directly by the faecal/oral route from an infected person to one who is not infected
          • indirectly by taking in the cysts in contaminated water or food when eating or drinking


          When hookworms get inside people, they lay their eggs inside the person's intestines. These eggs get into the soil or water when infected human faeces has been left on the ground or from faulty or broken sewage systems.

          Tiny larvae (young worms) will hatch out. If the soil is wet the larvae will develop to a stage where they can infect people. They can survive in wet soil for several weeks and are able to burrow through unbroken skin. This happens when people's skin comes into contact with water, soil or faeces which is infected with hookworm larvae.

          People can become infected with hookworm directly by the ingestion of larvae or by larvae burrowing through the skin.

          People in the tropical parts of northern Australia who walk around in contaminated wet places without shoes are very likely to get infected.

          Inside the body the larvae travel through the blood stream to the lungs where they are coughed up and then swallowed. They finally reach the intestines where they develop into adult worms. Adult worms are able to attach themselves to the walls of the intestines. They have hooks around the mouth which allow them to do this. They live there and suck blood from the human host.

          Fig. 1.14: How hookworm gets into the body and where it lives in the body.

          Threadworm (or pinworm)

          These worms look like tiny white threads and live in the intestine. The female worm will travel to the anal opening to lay its eggs on the skin around the anus. It is this activity which causes the itching. The eggs and the worms leave the body in faeces. The eggs hatch when they are taken into the same or another person's intestine.

          • directly through the faecal/oral route from an infected person to one who is not infected
          • indirectly through contact with contaminated clothing, bedding or food

          Dwarf tapeworm

          The dwarf tapeworm occurs in the stomach and intestines of humans. The adult tapeworm lays its eggs in the body. The eggs are passed out of the body in the faeces. If these eggs are ingested by other people indirectly or directly, the eggs will hatch in the intestine. The immature worm goes through two further stages of development before it becomes an adult.

          • directly by touching the mouth with fingers which are contaminated with faeces containing the egg
          • indirectly by ingesting eggs in contaminated food or water, or by swallowing an insect which has ingested eggs which have then hatched into larvae inside the insect


          Roundworms are nematodes and are found in northern parts of Australia and in many tropical countries. Strongyloides stercoralis is a roundworm which causes a life threatening disease called Strongyloidiasis.

          People can become infected through contact with soil contaminated by faeces containing the parasite.

          People can often get sick where hygiene and sanitation are poor. Infection can be detected with a special blood test and people can be cured with special tablets.


          These small animals are a type of mite. The female burrows into the skin where it lays its eggs. When the mites hatch they climb out onto the surface of the skin and then enter hair follicles. These are the small openings in the skin which hold the hair roots. The young mites grow into adults in the hair follicles. They then climb out and mate and start the process all over again. It is the burrowing activity of the mites which causes the skin irritation associated with scabies.

          Fig. 1.15: Scabies' life cycle.

          Scabies prefer to live in certain places in the body. These are body creases such as the backs of the knee and elbow and in the armpit and groin.

          Fig. 1.16: Scabies rash on the body.

          • direct contact or
          • indirect contact with contaminated clothing or bedding. Infection happens more frequently when people live in overcrowded conditions

          Head lice

          Adult lice live their whole lives in the hair of a person’s head. The lice stab openings in the skin to suck blood. The eggs of the head lice, which are also called nits, are glued to the hairs on the person's head. The nits are about 1 mm in size and are whitish in colour. They take about a week to hatch.

          New coronavirus may spread through poop

          The new coronavirus SARS-CoV-2, which has now infected nearly 76,000 people, spreads mostly through respiratory droplets and contact with infected patients. But new research suggests that it can also spread through feces.

          There are currently more cases of COVID-19 (the disease caused by the virus SARS-CoV-2) than would be expected if the virus were spreading only through respiratory droplets and contact with infected patients, according to a report published Feb. 15 by the Chinese Center for Disease Control and Prevention (China CDC).

          Previous tests have found that the coronavirus can be present in feces, but it was unclear if the virus would be viable enough to spread to another person, according to a previous Live Science report. So, a group of researchers analyzed stool samples from patients with COVID-19.

          They isolated the coronavirus from one patient who had severe pneumonia and examined the virus under an electron microscope. They found that the coronavirus was viable. "This means that stool samples may contaminate hands, food, water, etc.," the China CDC wrote in the report. People who use the bathroom and then don't wash their hands could spread the virus to others, for instance.

          "This virus has many routes of transmission, which can partially explain its strong transmission and fast transmission speed," the China CDC wrote. To avoid feces contamination, the China CDC recommends washing your hands frequently, disinfecting surfaces, maintaining personal hygiene, avoiding the consumption of raw food, boiling water before drinking it and disinfecting hospital environments.

          Another study, published Feb. 17 in the journal Emerging Microbes and Infections, found that the virus was present in blood and anal swabs taken from patients infected with SARS-CoV-2.

          With impressive cutaway illustrations that show how things function, and mindblowing photography of the world&rsquos most inspiring spectacles, How It Works represents the pinnacle of engaging, factual fun for a mainstream audience keen to keep up with the latest tech and the most impressive phenomena on the planet and beyond. Written and presented in a style that makes even the most complex subjects interesting and easy to understand, How It Works is enjoyed by readers of all ages.
          View Deal

          Not really news. Almost all disease viral or bacteriological can to some degree spread through fresh warm poop and even fresh cooling poop only hours old.

          Big news would be that it lasted days or weeks or could pass through rough primary sewage treatment plants to rivers (as might happen during heavy rainfall even in most US cities or in lesser developed countries where primary treatment was the high bar).

          Paper fails quite a bit as does where you hold it.

          Its distressing that you imply you see no need to use soap and water after pooping simply because you use toilet paper. Wink.

          Seriously water as water cleaning of posterior is SUPERIOR to toilet paper. Paper leaves smears. Bidets are more ecologically sound as well. Cheaper on plumbing clogs for household too.

          *** In all cases (toilet paper or bidet) you should use soap and water on hands with inspection and cleaning of fingernails. ***

          Paper fails quite a bit as does where you hold it.

          Its distressing that you imply you see no need to use soap and water after pooping simply because you use toilet paper. Wink.

          Seriously water as water cleaning of posterior is SUPERIOR to toilet paper. Paper leaves smears. Bidets are more ecologically sound as well. Cheaper on plumbing clogs for household too.

          *** In all cases (toilet paper or bidet) you should use soap and water on hands with inspection and cleaning of fingernails. ***

          Bidets are fairly standard through out the world barring North America and the UK. Even the UK is slowly transitioning to bidet availability and soap and water for hands afterward.

          Of course every national rivalry, mostly friendly or not, will probably make reference to ". except rival B where they do not wipe or wash at all. " to the other side.

          [email protected]
          It's you who implies that all Chinese use soap after touching their poop with bare hand, can you guarantee that they use soap afterwards? NO!

          You've said: "Paper fails quite a bit as does where you hold it. " -IT DOES if you are ignorant enough not to FOLD IT ONCE OR TWICE before use. :ROFLMAO::ROFLMAO::ROFLMAO:.
          Anyway, if it happens that I touch a bit of feces, then I wash my hands at least two times more than usual, because it leaves terrible stench.

          On the other hand, you are grabbing fistful of your warm poop :poop::poop:(n)(n) with your bare hands each time after defecation, like some filthy disgusting monkey. You must be stinking, and probably got used to the stench, same as many Chinese people stink, and I'm not trying to insult them that by saying that, it's just a fact.

          Questions & Answers: Sickness caused by E. coli

          E. coli is a common kind of bacteria that lives in the intestines of animals and people. There are many strains of E. coli. Most are harmless. However, one dangerous strain is called E. coli O157:H7. It produces a powerful poison. You can become very sick if it gets into your food or water.

          In 1999 it was estimated that about 73,000 people in the U.S. got sick each year from E. coli. About 60 died. It&rsquos believed that the number of illnesses and deaths has been dropping since then.

          How is E. coli O157:H7 spread?

          Outbreaks often are caused by food that has gotten the bacteria, E coli, in it. Bacteria can get accidentally mixed into ground beef before packaging. Eating undercooked meat can spread the bacteria, even though the meat looks and smells normal. E. coli can also live on cows&rsquo udders. It may get into milk that is not pasteurized.

          Raw vegetables, sprouts, and fruits that have been grown or washed in dirty water can carry E. coli O157:H7. It can get into drinking water, lakes, or swimming pools that have sewage in them. It is also spread by people who have not washed their hands after going to the toilet.

          E. coli can be spread to playmates by toddlers who are not toilet trained or by adults who do not wash their hands carefully after changing diapers. Children can pass the bacteria in their stool to another person for 2 weeks after they have gotten well from an E. coli O157:H7 illness. Older children and adults rarely carry the bacteria without symptoms.

          What are the signs of E. coli O157:H7 sickness?

          Bloody diarrhea and stomach pain are the most common signs of E. coli O157:H7 sickness. People usually do not have a fever, or may have only a slight fever.

          Some people, especially children under 5 and the elderly, can become very sick from E. coli O157:H7. The infection damages their red blood cells and their kidneys. This only happens to about 1 out of 50 people, but it is very serious. Without hospital care, they can die. See a doctor right away if you think you may have gotten sick from E. coli O157:H7.

          How will my doctor know if E. coli O157:H7 made me sick?

          Your doctor will test to see if your sickness was caused by E. coli by sending a stool sample to a lab. The lab will test for the bacteria.

          Anyone who suddenly has diarrhea with blood in it should call or see a doctor.

          How is it treated?

          Your doctor will tell you what is best. Taking medicine on your own may not help you get better, and it could make things worse. Do not take antibiotics or diarrhea medicine like Imodium® unless your doctor tells you to.

          Will E. coli O157:H7 infection cause problems for me later?

          People who have only diarrhea and stomach ache usually get completely well in 5-10 days. They do not have problems later.

          For those people who get very sick and have kidney failure, about 1 out of 3 may have kidney problems later. In rare cases, people have other problems like high blood pressure, blindness, or are paralyzed. Talk to your doctor if you have questions about this.

          What is the U.S. government doing to keep food safe from E. coli O157:H7?

          New laws have helped keep food from being contaminated with E. coli O157:H7. They keep meat safer during slaughter and grinding, and vegetables safer when they are grown, picked, and washed. But there is still a chance that E. coli O157:H7 could reach your food, so you should take the precautions listed below.

          What can I do to stay safe from E. coli O157:H7?

          • During an outbreak: Carefully follow instructions provided by public health officials on what foods to avoid in order to protect yourself and your family from infection.
          • Cook all ground beef thoroughly. During an outbreak of E. coli O157:H7, vegetables should be boiled for at least 1 minute before serving.
          • Cook ground beef to 160° F Test the meat by putting a food thermometer in the thickest part of the meat. Do not eat ground beef that is still pink in the middle.
          • If a restaurant serves you an under-cooked hamburger, send it back for more cooking. Ask for a new bun and a clean plate, too.
          • Don&rsquot spread bacteria in your kitchen. Keep raw meat away from other foods. Wash your hands, cutting board, counter, dishes, and knives and forks with hot soapy water after they touch raw meat, spinach, greens, or sprouts.
          • Never put cooked hamburgers or meat on the plate they were on before cooking. Wash the meat thermometer after use.
          • Drink only pasteurized milk, juice, or cider. Frozen juice or juice sold in boxes and glass jars at room temperature has been pasteurized, although it may not say so on the label.
          • Drink water from safe sources like municipal water that has been treated with chlorine, wells that have been tested or bottled water.
          • Do not swallow lake or pool water while you are swimming.

          Page last modified December 10, 2006
          Content source: CDC Clear and Cultural Communications


          An infectious disease agent can be transmitted in two ways: as horizontal disease agent transmission from one individual to another in the same generation (peers in the same age group) [2] by either direct contact (licking, touching, biting), or indirect contact through air – cough or sneeze (vectors or fomites that allow the transmission of the agent causing the disease without physical contact) [3] or by vertical disease transmission, passing the agent causing the disease from parent to offspring, such as in prenatal or perinatal transmission. [4]

          The term infectivity describes the ability of an organism to enter, survive and multiply in the host, while the infectiousness of a disease agent indicates the comparative ease with which the disease agent is transmitted to other hosts. [5] Transmission of pathogens can occur by direct contact, through contaminated food, body fluids or objects, by airborne inhalation or through vector organisms. [6]

          Transmissibility is the probability of an infection, given a contact between an infected host and a noninfected host. [7]

          Community transmission means that the source of infection for the spread of an illness is unknown or a link in terms of contacts between patients and other people is missing. It refers to the difficulty in grasping the epidemiological link in the community beyond confirmed cases. [8] [9] [10]

          Local transmission means that the source of the infection has been identified within the reporting location (such as within a country, region or city). [11]

          The route of transmission is important to epidemiologists because patterns of contact vary between different populations and different groups of populations depending on socio-economic, cultural and other features. For example, low personal and food hygiene due to the lack of a clean water supply may result in increased transmission of diseases by the fecal-oral route, such as cholera. Differences in incidence of such diseases between different groups can also throw light on the routes of transmission of the disease. For example, if it is noted that polio is more common in cities in underdeveloped countries, without a clean water supply, than in cities with a good plumbing system, we might advance the theory that polio is spread by the fecal-oral route. Two routes are considered to be airborne: Airborne infections and droplet infections. [ citation needed ]

          Airborne infection Edit

          "Airborne transmission refers to infectious agents that are spread via droplet nuclei (residue from evaporated droplets) containing infective microorganisms. These organisms can survive outside the body and remain suspended in the air for long periods of time. They infect others via the upper and lower respiratory tracts." [12] The size of the particles for airborne infections need to be < 5 μm. [13] It includes both dry and wet aerosols and thus requires usually higher levels of isolation since it can stay suspended in the air for longer periods of time. i.e., separate ventilation systems or negative pressure environments are needed to avoid general contamination. e.g., tuberculosis, chickenpox, measles. [ citation needed ]

          A common form of transmission is by way of respiratory droplets, generated by coughing, sneezing, or talking. Respiratory droplet transmission is the usual route for respiratory infections. Transmission can occur when respiratory droplets reach susceptible mucosal surfaces, such as in the eyes, nose or mouth. This can also happen indirectly via contact with contaminated surfaces when hands then touch the face. Before drying, respiratory droplets are large and cannot remain suspended in the air for long, and are usually dispersed over short distances. [12] The size of the particles for droplet infections are > 5 μm. [13]

          Direct contact Edit

          Direct contact occurs through skin-to-skin contact, kissing, and sexual intercourse. Direct contact also refers to contact with soil or vegetation harboring infectious organisms. [17] Additionally, while fecal–oral transmission is primarily considered an indirect contact route, direct contact can also result in transmission through feces. [18] [19]

          Diseases that can be transmitted by direct contact are called contagious (contagious is not the same as infectious although all contagious diseases are infectious, not all infectious diseases are contagious). These diseases can also be transmitted by sharing a towel (where the towel is rubbed vigorously on both bodies) or items of clothing in close contact with the body (socks, for example) if they are not washed thoroughly between uses. For this reason, contagious diseases often break out in schools, where towels are shared and personal items of clothing accidentally swapped in the changing rooms. [ citation needed ]

          Some diseases that are transmissible by direct contact include athlete's foot, impetigo, syphilis, warts, and conjunctivitis. [20]

          Sexual Edit

          This refers to any disease that can be caught during sexual activity with another person, including vaginal or anal sex or (less commonly) through oral sex (see below). Transmission is either directly between surfaces in contact during intercourse (the usual route for bacterial infections and those infections causing sores) or from secretions (semen or the fluid secreted by the excited female) which carry infectious agents that get into the partner's blood stream through tiny tears in the penis, vagina or rectum (this is a more usual route for viruses). In this second case, anal sex is considerably more hazardous since the penis opens more tears in the rectum than the vagina, as the vagina is more elastic and more accommodating. [ citation needed ]

          Oral sexual Edit

          Sexually transmitted diseases such as HIV and hepatitis B are thought to not normally be transmitted through mouth-to-mouth contact, although it is possible to transmit some STDs between the genitals and the mouth, during oral sex. In the case of HIV this possibility has been established. It is also responsible for the increased incidence of herpes simplex virus 1 (which is usually responsible for oral infections) in genital infections and the increased incidence of the type 2 virus (more common genitally) in oral infections. [ citation needed ]

          Oral Edit

          Diseases that are transmitted primarily by oral means may be caught through direct oral contact such as kissing, or by indirect contact such as by sharing a drinking glass or a cigarette. Diseases that are known to be transmissible by kissing or by other direct or indirect oral contact include all of the diseases transmissible by droplet contact and (at least) all forms of herpes viruses, namely Cytomegalovirus infections herpes simplex virus (especially HSV-1) and infectious mononucleosis. [ citation needed ]

          Mother-to-child transmission Edit

          This is from mother to child (more rarely father to child), often in utero, during childbirth (also referred to as perinatal infection) or during postnatal physical contact between parents and offspring. In mammals, including humans, it occurs also via breast milk (transmammary transmission). Infectious diseases that can be transmitted in this way include: HIV, hepatitis B and syphilis. Many mutualistic organisms are transmitted vertically. [21]

          Iatrogenic Edit

          Transmission due to medical procedures, such as touching a wound, an injection or transplantation of infected material. Some diseases that can be transmitted iatrogenically include: Creutzfeldt–Jakob disease by injection of contaminated human growth hormone, MRSA and many more. [ citation needed ]

          Indirect contact Edit

          Indirect contact transmission, also known as vehicleborne transmission, involves transmission through contamination of inanimate objects. Vehicles that may indirectly transmit an infectious agent include food, water, biologic products such as blood, and fomites such as handkerchiefs, bedding, or surgical scalpels. A vehicle may passively carry a pathogen, as in the case of food or water may carrying hepatitis A virus. Alternatively, the vehicle may provide an environment in which the agent grows, multiplies, or produces toxin, such as improperly canned foods provide an environment that supports production of botulinum toxin by Clostridium botulinum. [17]

          Transmission by other organisms Edit

          A vector is an organism that does not cause disease itself but that transmits infection by conveying pathogens from one host to another. [22]

          Vectors may be mechanical or biological. A mechanical vector picks up an infectious agent on the outside of its body and transmits it in a passive manner. An example of a mechanical vector is a housefly, which lands on cow dung, contaminating its appendages with bacteria from the feces, and then lands on food prior to consumption. The pathogen never enters the body of the fly. In contrast, biological vectors harbor pathogens within their bodies and deliver pathogens to new hosts in an active manner, usually a bite. Biological vectors are often responsible for serious blood-borne diseases, such as malaria, viral encephalitis, Chagas disease, Lyme disease and African sleeping sickness. Biological vectors are usually, though not exclusively, arthropods, such as mosquitoes, ticks, fleas and lice. Vectors are often required in the life cycle of a pathogen. A common strategy used to control vector-borne infectious diseases is to interrupt the life cycle of a pathogen by killing the vector. [ citation needed ]

          Fecal–oral Edit

          In the fecal-oral route, pathogens in fecal particles pass from one person to the mouth of another person. Although it is usually discussed as a route of transmission, it is actually a specification of the entry and exit portals of the pathogen, and can operate across several of the other routes of transmission. [17] Fecal–oral transmission is primarily considered as an indirect contact route through contaminated food or water. However, it can also operate through direct contact with feces or contaminated body parts, such as through anal sex. [18] [19] It can also operate through droplet or airborne transmission through the toilet plume from contaminated toilets. [23] [24]

          Main causes of fecal–oral disease transmission include lack of adequate sanitation and poor hygiene practices - which can take various forms. Fecal oral transmission can be via foodstuffs or water that has become contaminated. This can happen when people do not adequately wash their hands after using the toilet and before preparing food or tending to patients. [ citation needed ]

          The fecal-oral route of transmission can be a public health risk for people in developing countries who live in urban slums without access to adequate sanitation. Here, excreta or untreated sewage can pollute drinking water sources (groundwater or surface water). The people who drink the polluted water can become infected. Another problem in some developing countries, is open defecation which leads to disease transmission via the fecal-oral route. [ citation needed ]

          Even in developed countries there are periodic system failures resulting in a sanitary sewer overflow. This is the typical mode of transmission for infectious agents such as cholera, hepatitis A, polio, Rotavirus, Salmonella, and parasites (e.g. Ascaris lumbricoides). [ citation needed ]

          Tracking the transmission of infectious diseases is called disease surveillance. Surveillance of infectious diseases in the public realm traditionally has been the responsibility of public health agencies, either on the (inter)national or a local level. Public health staff relies on health care workers and microbiology laboratories to report cases of reportable diseases to them. The analysis of aggregate data can show the spread of a disease and is at the core of the specialty of epidemiology. To understand the spread of the vast majority of non-notifiable diseases, data either need to be collected in a particular study, or existing data collections can be mined, such as insurance company data or antimicrobial drug sales for example. [ citation needed ]

          For diseases transmitted within an institution, such as a hospital, prison, nursing home, boarding school, orphanage, refugee camp, etc., infection control specialists are employed, who will review medical records to analyze transmission as part of a hospital epidemiology program, for example. [ citation needed ]

          Because these traditional methods are slow, time-consuming, and labor-intensive, proxies of transmission have been sought. One proxy in the case of influenza is tracking of influenza-like illness at certain sentinel sites of health care practitioners within a state, for example. [25] Tools have been developed to help track influenza epidemics by finding patterns in certain web search query activity. It was found that the frequency of influenza-related web searches as a whole rises as the number of people sick with influenza rises. Examining space-time relationships of web queries has been shown to approximate the spread of influenza [26] and dengue. [27]

          Computer simulations of infectious disease spread have been used. [28] Human aggregation can drive transmission, seasonal variation and outbreaks of infectious diseases, such as the annual start of school, bootcamp, the annual Hajj etc. Most recently, data from cell phones have been shown to be able to capture population movements well enough to predict the transmission of certain infectious diseases, like rubella. [29]

          Pathogens must have a way to be transmitted from one host to another to ensure their species' survival. Infectious agents are generally specialized for a particular method of transmission. Taking an example from the respiratory route, from an evolutionary perspective viruses or bacteria that cause their host to develop coughing and sneezing symptoms have a great survival advantage, as they are much more likely to be ejected from one host and carried to another. This is also the reason that many microorganisms cause diarrhea. [ citation needed ]

          The relationship between virulence and transmission is complex and has important consequences for the long term evolution of a pathogen. Since it takes many generations for a microbe and a new host species to co-evolve, an emerging pathogen may hit its earliest victims especially hard. It is usually in the first wave of a new disease that death rates are highest. If a disease is rapidly fatal, the host may die before the microbe can be passed along to another host. However, this cost may be overwhelmed by the short term benefit of higher infectiousness if transmission is linked to virulence, as it is for instance in the case of cholera (the explosive diarrhea aids the bacterium in finding new hosts) or many respiratory infections (sneezing and coughing create infectious aerosols). [ citation needed ]

          The mode of transmission is also an important aspect of the biology of beneficial microbial symbionts, such as coral-associated dinoflagellates or human microbiota. Organisms can form symbioses with microbes transmitted from their parents, from the environment or unrelated individuals, or both. [ citation needed ]

          Vertical transmission Edit

          Vertical transmission refers to acquisition of symbionts from parents (usually mothers). Vertical transmission can be intracellular (e.g. transovarial), or extracellular (for example through post-embryonic contact between parents and offspring). Both intracellular and extracellular vertical transmission can be considered a form of non-genetic inheritance or parental effect. It has been argued that most organisms experience some form of vertical transmission of symbionts. [30] Canonical examples of vertically transmitted symbionts include the nutritional symbiont Buchnera in aphids (transovarially transmitted intracellular symbiont) and some components of the human microbiota (transmitted during passage of infants through the birth canal and also through breastfeeding). [ citation needed ]

          Horizontal transmission Edit

          Some beneficial symbionts are acquired horizontally, from the environment or unrelated individuals. This requires that host and symbiont have some method of recognizing each other or each other's products or services. Often, horizontally acquired symbionts are relevant to secondary rather than primary metabolism, for example for use in defense against pathogens, [31] but some primary nutritional symbionts are also horizontally (environmentally) acquired. [32] Additional examples of horizontally transmitted beneficial symbionts include bioluminescent bacteria associated with bobtail squid and nitrogen-fixing bacteria in plants.

          Mixed-mode transmission Edit

          Many microbial symbionts, including human microbiota, can be transmitted both vertically and horizontally. Mixed-mode transmission can allow symbionts to have the “best of both worlds” – they can vertically infect host offspring when host density is low, and horizontally infect diverse additional hosts when a number of additional hosts are available. Mixed-mode transmission make the outcome (degree of harm or benefit) of the relationship more difficult to predict, because the evolutionary success of the symbiont is sometimes but not always tied to the success of the host. [21]


          feeds on human food such as sugar, palm wine, rice
          , garri etc. when on the food, it vomits some juice into it. This juice is later taken back. It is during the feeding that the fly contaminates food with disease germs. The housefly is also very hairy and these hairs collect a lot of germs infested dirts which are left on food during their visits.

          Why disgust matters

          The new synthesis about disgust is that it is a system that evolved to motivate infectious disease avoidance. There are vital practical and intellectual reasons why we need to understand disgust better. Practically, disgust can be harnessed to combat the behavioural causes of infectious and chronic disease such as diarrhoeal disease, pandemic flu and smoking. Disgust is also a source of much human suffering it plays an underappreciated role in anxieties and phobias such as obsessive compulsive disorder, social phobia and post-traumatic stress syndromes it is a hidden cost of many occupations such as caring for the sick and dealing with wastes, and self-directed disgust afflicts the lives of many, such as the obese and fistula patients. Disgust is used and abused in society, being both a force for social cohesion and a cause of prejudice and stigmatization of out-groups. This paper argues that a better understanding of disgust, using the new synthesis, offers practical lessons that can enhance human flourishing. Disgust also provides a model system for the study of emotion, one of the most important issues facing the brain and behavioural sciences today.

          1. Introduction

          The premise of the new synthesis about disgust is that it is an adaptive system that evolved to motivate disease-avoidance behaviour [1–7]. It arose in our animal ancestors to facilitate the recognition of objects and situations associated with risk of infection and to drive hygienic behaviour, thus reducing micro- and macro-parasite contact. Sometime in our evolution towards human ultrasociality, disgust took on an extended role—providing a motive to punish antisocial behaviour and to shun the breakers of social rules [8,9]. Disgust is an adaptive system whereby individual responses vary according to an individual's personality and learning experience, as well as by local cultural effects such as norms about manners and the symbolism of pollution and purity [7]. This new synthesis replaces previous conceptions of disgust as, for example, a Freudian means of repudiating desired objects, such as the mother's breast or faeces [10], a psychodynamic balancing mechanism to refuse reminders of our animal nature [6,11,12] or as a social and cultural construction [13,14].

          But why, in the end, does it matter that we understand how and why disgust evolved? Disgust impacts on many aspects of our lives, from our individual, domestic, everyday hygiene habits, through our moral choices as members of society, to public policy on issues such as health, justice, social exclusion and warfare. However, possibly because it is the part of our nature that deals with repulsion, disgust has received scant scientific attention until recently [10]. This special issue demonstrates how disgust is now proving a fertile ground for study by psychologists, zoologists and evolutionary biologists. Beyond the life sciences, it also provides rich matter for the humanities—in the social sciences, in history and classical studies, in politics, jurisprudence and marketing, as well as in the arts.

          Clearly, the better we understand how and why disgust evolved and the part that it plays in our natures and in our societies, the better we will advance in all of these fields of intellectual endeavour. Such advances are important in themselves, but they also have practical repercussions. In this paper, I argue that there are three principal practical reasons why we need to better understand the biology of this ‘dark side’ of our nature.

          First, as one of our principal defences against infection, disgust can be harnessed to efforts at improving health. It can be employed in programmes to prevent diarrhoeal diseases, pandemic flu and to aid smoking cessation, for example. Second, disgust has important implications for psychological welfare. It plays a role in obsessive compulsive and post-traumatic stress disorders (OCD and PTSD) and it is part of the emotional cost of caring for the sick, elderly and infirm. Stigmatization and self-directed disgust cause suffering in conditions such as obesity and fistula. Thirdly, disgust is a moral emotion that influences social behaviours. Its role in religion, justice, technological progress, caste, class, xenophobia and the politics of exclusion needs to be better understood if we are to create healthier and more humane societies.

          Here I tackle each of these issues in turn and then draw out some of the questions that remain to be answered about this powerful but, still poorly understood emotion.

          2. Disgust and disease control

          The proper domain [15] of disgust is the avoidance of infectious disease [1]. Despite major recent improvements in our understanding of the transmission, prevention and treatment of infectious disease, the problem is still with us. Table 1 presents the major current and recent infectious disease threats to humans. Six conditions cause most deaths: diarrhoeal diseases, acute respiratory tract infections, malaria, measles, HIV and tuberculosis. Parasitic worms including schistosomes, hookworm, ascaris and the nematodes that cause lymphatic filariasis and oncocerciasis still infect one-third of the world's population [16]. Leprosy, polio, smallpox, plague and guinea worm were major causes of death and disability in previous centuries they are now rare or eradicated, thanks to recent control efforts [17]. Measles, diphtheria and meningitis also cause far less mortality owing to recent advances in vaccination. Newly emerging or re-emerging infections such as Ebola, SARS, West Nile and Rift Valley fevers and pandemic influenza are a major cause for concern, as is the emergence of resistance to antibiotics and antimalarials. Not included in the table are the infections that have also been shown to play a role in many chronic diseases, including cancer, stroke, multiple sclerosis and cardiovascular disease [18].

          Table 1. Disgust, behaviour and the major causes of infectious disease.

          While medical effort and attention has focused on the pathology of disease, and the search for vaccines and cures, measures to prevent the acquisition of infection in the first place have received less attention. Yet, as the table shows, avoidance behaviour is essential to prevent the spread of all of these conditions [17]. If hygiene is defined as disease-avoidance behaviour [19], then hygienic measures help to defend all of the principal portals of entry to the body. Safe excreta disposal, hand, food and water hygiene prevent the faecal–oral transmission of the diarrhoeal diseases including cholera, salmonelloses, as well as hepatitis A and E, polio and various worm infections. Avoiding sex with infected others helps prevent the transmission of HIV, syphilis and hepatitis B and C. Diseases that use the respiratory route such as tuberculosis, measles, influenza leprosy, diphtheria and respiratory tract infections are harder to prevent, but reducing proximity and contact with the sick hinders airborne transmission and the avoidance of contaminated fomites can help reduce infection risk [20]. Staphylococcal, streptococcal and tetanus infections can be prevented through body surface hygiene, especially by avoiding fluid transfer from and to skin lesions and from fomites. The body surface is also the route of injection of the infectious diseases that are carried by insect vectors, including malaria, onchocerciasis, leishmaniasis, typhus and yellow fever. Here, disease prevention means avoiding insect bites. Other vector-borne infections including rabies and toxoplasmosis can be prevented by avoiding contact with bats, rats, dogs and cats. A number of these infections have multiple routes of infection, especially the diseases of crowding (measles and tuberculosis).

          The final column of table 1 picks out items that have been cited as disgusting in various studies [1,2]. There are a variety of disgust elicitors that relate to almost every infectious condition. For the faecal–oral infections, these include faeces, dirty water and contaminated foods for the skin contact infections, skin lesions for sexually transmitted diseases, ulcerated genitals and individuals who may be at high risk such as sex workers for the respiratory infections, respiratory secretions and contaminated materials. Sick people and bodily secretions/excretions occasion disgust as the source of multiple possible infections. We have suggested elsewhere that most disgust elicitors can be incriminated in the transmission of infection from source to host and that this explains why they are found to be disgusting [2]. Those with lower disgust sensitivity are known to suffer from more infectious disease [21], and selective partner choice is an important, but underappreciated, factor in the spread of sexually transmitted infections [22].

          Of course, the diseases of recent centuries may not be a perfect proxy for the diseases that shaped the disgust response in our pre-human and human evolutionary history. Diseases with their origins in the domestication of animals or in high-density urban settlement, for example, are thought to be more prevalent now than in ancestral times [23]. Nevertheless, the table shows a general pattern whereby hygienic behaviour with respect to disgust elicitors plays an essential role in the prevention of infection. These behaviours are ancient and ubiquitous, many of them are shared with our animal ancestors [24] and are not contingent on recent scientific knowledge about the behaviour of the agents of infectious disease. Indeed, the idea of contacting or consuming infectious substances such as saliva, faeces or vomit, or of intimate contact with those known to be carrying infection is deeply uncomfortable to even contemplate. Self-limitation of such behaviour is so automatic and intuitive that it is often ignored as the front-line in our defence against disease.

          Without disgust and the hygienic behaviours it elicits, then, infectious diseases would cause far more morbidity and mortality in our own—and in all free-living animal—species. (There is one notable exception to this pattern. No disgust elicitor is involved in the insect-vectored infections such as malaria and oncocerciasis. Perhaps the adaptive response to a bite is not disgust, but to slap away the offending insect, or, alternatively, perhaps ancestral conditions were such that it was impossible to gain an adaptive advantage from insect bite-avoidance behaviour [1].)

          Disgust therefore plays a major role in public health. How can this knowledge be exploited in programmes to control disease? Where disgust reactions are appropriate to modern conditions they can be elicited. In cases where they are inappropriate, efforts can be made at redirection. Further, disgust can also be employed to help improve health beyond the domain of infectious disease.

          Take, for example, the diseases that are transmitted via the faecal–oral route. Though the situation is improving, diarrhoeal diseases still kill an estimated 1.5 million children every year [25]. Human faeces are the main source of infection [26]. Evidence suggests that handwashing with soap, were it practised globally, could save over a million lives a year, mainly from the infectious enteric diseases [27]. It can also prevent respiratory infections [28], including pandemic flu [20], infectious blinding trachoma [29], AIDS-associated infections [30] and potentially reduce malnutrition [31]. Handwashing with soap is, however, a rare practice. Direct observation showed that only 3 per cent of mothers in Ghana, 4 per cent in Madagascar, 12–14% in China, Tanzania and Uganda and 18 per cent in Kyrgyzstan [32] were washing their hands with soap after using the toilet. In the UK, we found that only 43 per cent of mothers washed their hands with soap after changing a dirty nappy [33] and electronic sensors showed that only 32 per cent of men and 64 per cent of women washed their hands with soap after using a public toilet [34]. Formative research studies into the reasons why people washed their hands found motives that included comfort, nurture, status and attraction. However, disgust at the idea that faecal material might be present on hands was consistently reported as the most powerful motivator of handwashing with soap after going to the toilet [32]. This information was employed in the development of a national handwashing campaign in Ghana. TV and radio commercials were designed to graphically highlight the contamination of hands and to show how invisible matter could be transferred to foods that were being eaten by children [35]. The campaign improved nationally reported rates of handwashing with soap by 13 per cent after the toilet and by 41 per cent before eating [36]. Similar improvements in hand hygiene were achieved in a social marketing campaign in Burkina Faso that used disgust-based messages, amongst others [37]. An image of a bacteria-ridden hand used as a screen saver in a Los Angeles hospital reportedly improved staff handwashing practices dramatically [38].

          Disgust has been evoked to promote handwashing in more controlled conditions. Porzig-Drummond et al. showed that adding disgust-relevant images to educational films and posters improved handwashing rates above the effect of education alone both in the laboratory and in the public washrooms [39]. Judah et al. displayed a variety of messages at the entrance to a public toilet and found that disgust-based messages such as ‘soap it off or eat it later’ were among the most effective in increasing soap use, especially in men [34].

          Disgust featured in the UK Government response to the threat of a pandemic of H1N1 influenza in 2009/2010. The cover image of an information leaflet delivered to every household in the UK (see [41]) to raise awareness of hand and respiratory hygiene explicitly depicts the aerosol spread of bodily secretions in a sneeze coming directly towards the viewer. Exposure to the campaign material was associated with increases in hygienic behaviour, such as the purchase of antibacterial hand gel, though the specific effect of the disgust component was not explicitly evaluated [29].

          Mass media is, of course, not the only source of individual learning about disgust and hygiene. Children throughout the world are socialized into hygiene rules at an early age by parents, family, school and the wider community [7,41]. Children learn to bathe and groom themselves, especially before social interaction, to avoid sharing toilet articles, to avoid wearing night clothes in public, where (and where not) to defecate, to use a handkerchief and to eat ‘politely’ without exchange of bodily fluids. Individuals who do not display continence with their own emanations are regarded as having ‘bad manners’ and are denied the benefits of social interaction [42]. Having well-mannered children is an important aspiration for mothers in most societies [32]. Though the subject has been little investigated, it appears that mothers recruit disgust to teach their children how to behave, pulling disgust faces and making appropriate ‘yuk’ noises as children ‘make messes’. The process is aided by a predisposition or preparedness [43–45] to learn disgust of bodily fluids. The fact that failing to display continence with bodily fluids is socially unacceptable was exploited in a graphic television commercial for the Florida Department of Health, where one character sneezes over food, surface, hands and workmates, causing colleagues to demonstrate their disgust at these lapses in respiratory manners [46]. In our public toilet study using unobtrusive sensors, we found that rates of handwashing decreased when there were few people in the facility, and the message that had the biggest positive effect was: ‘Is the person next to you washing hands with soap?’ [34]. Manners are a subject about which almost nothing has been written in the academic press, yet understanding them better might prove fruitful in the quest to prevent the person-to-person spread of infection.

          Whilst disgust of nasal emanations and of people who spread them is probably an appropriate and adaptive reaction to the threat of pandemic influenza [47], disgust responses can sometimes be unhelpful to public health. The disgust system operates by the precautionary principle whereby it is better to miss one meal than to run the risk of contracting a life-threatening disease [48,49]. Hence responses can be disproportionate to actual risk. The food industry is regularly affected by contamination scares that can lead to huge, but temporary shifts away from purchasing meat products, eggs or chocolate, for example [50]. The French public reduced their consumption of beef in response to emotive stories in the press about what could happen if you ate ‘mad cow’ [51]. A study of public reactions to a hypothetical outbreak of pneumonic plague found that people were likely to want to avoid health centres, when attending would have been beneficial to their health [52]. In California, public protestations of disgust have derailed projects for converting wastewater into drinking water [53].

          While disgust's proper domain is that of infectious disease, it has also been employed in efforts to tackle other public health problems, most notably smoking. Cessation campaigns have used disgust freely. For example, the British Heart Foundation's most successful media campaign entitled ‘Give up before you clog up’ graphically depicted the impact of smoking on arteries by showing cigarettes apparently dripping globs of fat [54]. The World Health Organization now recommends the use of graphic images of diseased organs on cigarette packs. A Canadian study showed that the greater the disgust reported at such pictures, the greater the likelihood that participants would have attempted to, or succeeded in, quitting [55].

          Disgust appears to have played a role in what Rozin calls the ‘moralization’ of smoking, which has become disgusting because it has been linked to contamination and disease. Individuals then display strong aversion to even minimal contact with the offensive substance (refusing smokers hotel rooms, for example). Rozin notes, ‘when disgust becomes linked to an entity or activity, rejection or avoidance of that activity becomes highly motivated and internalized’. He found that moral reactions to smoking depended more on how disgusting than on how unhealthy it was perceived to be [56]. The modern practice of relegating smokers to the outside of public buildings reinforces the rejection of the disgusting practice and the stigmatization of the individual as antisocial.

          Food-related disgust can be employed to combat obesity. In a UK TV show, all of the snack and junk food typically eaten by one school class was piled into one huge disgusting heap to highlight the poor quality of children's diets [57]. This helped to spur the UK government to improve the quality of school dinners. Variations on this idea, such as graphic displays of the fat and sugar content of common food items, could help to harness disgust to healthy eating campaigns. One could imagine further uses of disgust targeted at obesity or at unsafe sex, for example. However, disgust has to be employed responsibly, as it can encourage moralization and stigmatization, as we have seen.

          3. Disgust and psychological welfare

          Disgust is a strong and visceral emotion that can arouse powerful affective and behavioural responses. While the emotion arose to defend against infectious disease, it can also cause maladaptive behaviour, interfering with the ability to lead a normal life. Some problems are associated with pathologies of the disgust system. Others may be due to disgust's ‘normal’ functioning in the context of an abnormal or novel environment. Further, some professions require the suppression of disgust, which could be regarded as a psychological cost that has to be borne by those individuals—and by society as a whole.

          Disgust sensitivity varies from one individual to another along a continuum [58]. We might expect then that individuals at the very high, or very low ends of the spectrum might manifest behavioural problems associated with being too easily or too little disgusted. Those who are too easily disgusted might be predicted to manifest phobias associated with potential disease sources such as other people, body products, sexual organs and by-products, certain foodstuffs and disease-related animals. Those who are, on the contrary, too little disgusted might find difficulty in being accepted into society and in maintaining bodily and domestic hygiene, with implications for their own health and that of their dependants. Unfortunate disgust experiences might also leave unpleasant or debilitating sequelae including post traumatic stress disorder (PTSD).

          How far does the evidence bear out these predictions? A number of studies suggest that some forms of obsessive compulsive disorder (OCD) can best be understood as disorders of the disgust system [59]. Up to 50 per cent of OCD patients present with contamination fears [60]. They suffer from intrusive thoughts of contamination and impurity and reduce their distress by excessive sanitation and disinfection of the self and the environment [61]. These patients tend to rate contaminated objects as ‘disgusting’ rather than ‘frightening’ [62]. In one memorable experiment, Tolin and colleagues created a chain of contagion where a pencil was touched to a toilet bowl and then wiped on another pencil, and that one onto another in sequence. ‘Normal’ participants, and those with chronic anxiety, reported diminishing contamination that had largely disappeared by the fourth pencil. However, the OCD patients reported appreciable contamination even beyond the tenth pencil. They described a world of spreading, looming vulnerability where they cannot control the threat of contagion [62]. As OCD occurs along a continuum of severity, it is likely that for every individual who seeks help, there will be many more borderline individuals who suffer from some form of debilitating contamination anxiety.

          It might be expected that contamination anxiety would become more severe when disease reminders become more prevalent, for example, during epidemics [63]. Indeed, contamination fears and washing rituals were exacerbated in OCD patients during the recent H1N1 swine flu pandemic [64]. Planners need to bear in mind the possible social costs of pandemic awareness campaigns and consider the need for additional support services.

          As ultrasocial beings, humans depend on others for survival, yet others are also the main source of infectious disease. Overactive disgust responses may play a role in some forms of social phobia. Though most goes unreported, at any one time 4.5 per cent of Americans may be suffering from social phobias and 2.3 per cent from agoraphobia [65]. Symptoms of both include an abnormal unwillingness to venture into crowds and to contact other people. The evidence is equivocal as to whether disgust plays a role—while agoraphobics have heightened disgust sensitivity [66] and agoraphobia is twice as common in women than it is in men (consistent with female disgust sensitivity being higher on average than it is for men [2]), one study found no heightened disgust sensitivity in social phobia (possibly because an instrument measuring only food-related disgust was employed) [66].

          A variety of other debilitating specific phobias are also candidates for pathologies of the disgust system. Blood-injection-injury phobia is characterized by extreme aversion to the sight of blood, injuries, or surgical procedures including injections. Sufferers have higher disgust sensitivity, rate disgusting images as more disgusting than controls and display stronger facial expressions of disgust [67]. A variety of small animal and insect phobias are also potentially disgust-related. Animals that have connections with disease and dirt are much more likely candidates for phobias and childhood fears than those that do not (e.g. spiders, rats, worms, maggots, cockroaches and teeming insects [68]). Recent research also suggests that disgust is a stronger predictor than anxiety of spider avoidance [69]. Trichotillomania may also be disgust-related pulling out skin hairs may be an exaggerated response to the possible presence of ectoparasites in skin—a hypothesis that has some support in the literature [70].

          Clinical observation suggests that disgust is a primary feature of eating disorders such as anorexia and bulimia [10]. While some studies have shown correlations between measures of disgust sensitivity and of eating disorders [71,72]), others have failed to find such associations [66,73]. Disgust with the shape of one's own body is often a feature of eating disorders, and indeed obesity does tend to be seen as disgusting [74,75]. Meat is one of the most likely sources of pathogens in food and is also a particular focus of food-related disgust. It is therefore unsurprising that most cultures have taboos about what meats are suitable to eat and many cultures and sub-cultures such as Hindus and vegetarians/vegans reject it entirely [76].

          As sexual acts, body parts and products are a focus of disgust, one might expect pathologies of the disgust system to affect sexual function. Though the problem has been little studied, several authors report that disgust is implicated in undermining sexual arousal and desire [77,78]. De Jong et al. [77] present case studies of women who had turned disgust on themselves associating dirt, disease, fistula and defaecation problems with their vagina which led to an inability to face intercourse.

          Disgust may also play a role in the decision to remain celibate. The problem is described by one ex-celibate, the UK broadcaster Stephen Fry:

          ‘I would be greatly in the debt of the man who could tell me what would ever be appealing about those damp, dark, foul-smelling and revoltingly tufted areas of the body that constitute the main dishes in the banquet of love… …Once under the influence of the drugs supplied by one's own body, there is no limit to the indignities, indecencies, and bestialities to which the most usually rational and graceful of us will sink’ [79].

          If the psychological problems we have discussed above are pathologies of the disgust system, then comorbidities are also to be expected. Monteiro et al. [80] found that 24 per cent of patients with untreated OCD were virgins and another 9 per cent had not been sexually active for years. Of their 25 patients, seven who reported sexual problems also suffered from extreme shyness, suggesting possible social phobia comorbidity with OCD.

          If sexual, social, contamination, blood injury and food-related phobias may, at least partly, be explained as maladaptive disorders of an evolved disease-avoidance system, another class of disgust-related psychological disorders can be classed as adaptive responses to a hostile environment. In particular, it seems that one class of PTSD may result from extreme experiences associated with disgust. Olatunji et al. [81] showed that rape victims with PTSD suffer from feelings of dirtiness associated with mental pollution. Victims of childhood sexual abuse and survivors of torture may suffer in similar ways. Dalgleish and Power provide case histories where extremely disgusting events such as encounters with decomposed corpses in war or at work, or biological contaminants in food lead to intrusive thoughts, flashbacks, recurrent nausea, feelings of dirtiness that cannot be removed by washing and other manifestations that can leave patients unable to lead a normal life [82].

          If the clinical and subclinical conditions that I have described are indeed disgust system disorders, then practical implications follow. First of all, accurate diagnosis is required, and seeing such problems as potentially disgust-related can help to hone the instruments of diagnosis. Secondly, many of these conditions occur on a sliding scale in the population, and many are associated with shame and an extreme reluctance to disclose or present to health services hence much suffering goes undiagnosed and unaided. Health workers need to be well-briefed to detect hints of these conditions and to look for comorbidities, for example, for sexual dysfunction in those presenting with OCD. Internet-based support for such conditions may be more acceptable to many than face-to-face interaction [77].

          Thirdly, there are many approaches to treatment both through behavioural and drug therapies. A systematic look at what has worked in each of these conditions through the lens of disgust might reveal effective therapies. For example, we know that cognitive reappraisal is possible. Just as rotting milk can be relabelled as yoghurt and so becomes palatable, de Jong suggests that exercises aimed at reconstructing sexual organs, not as smelly and dirty, but as examples of exquisite design could be effective in reducing sexual phobias [83]. Work is needed to determine whether behavioural therapies such as Exposure with Response Prevention and microbiological experiments demonstrating the lack of organisms on objects perceived to be contaminated [62] are effective. Cognitive behavioural therapy involving habituation to disgust objects and extinction along with the formation of new and positive associations could be used across these phobias, possibly with the addition of cortisol, which has been shown to enhance the consolidation of newly learnt memories [77]. Drug therapies might also focus on the possible implication of serotonin pathways in disgust [84].

          Finally, research suggests that the disgust system is made up of a series of components that relate to different types of disease threat (sexual, hygiene, blood-and-guts, food, sick people, animals/insects, etc. [85]). It seems likely that each type of threat has its own type of phobia. The disgust scales in current use [86–88] are based on psychodynamic conceptions of disgust that predate the new evolutionary synthesis [89] or do not distinguish between types of organic disgust [6]. We are currently preparing a new scale based on the discrete disease-avoidance tasks of disgust which should have more power to help in distinguishing the discrete pathologies of disgust subsystems.

          4. The social uses and abuses of disgust

          While disgust is the primary means by which individual humans detect and avoid infectious pathogens, the problem is not just an individual one. Parasites tend to specialize in exploiting the particular biochemical and morphological features of their hosts, making parasite transmission most likely between biologically similar organisms. Social animals thus face a conundrum sociality brings fitness benefits, but at the same time it carries an elevated risk of infectious disease. For an ultrasocial species, such as humans, the problem is more acute, as parasites adapt to take advantage of sustained social proximity and interaction. Individuals have to protect themselves and their kin from parasites that have evolved to take every transmission opportunity. Appropriate disease-avoidance strategies thus include preferring to mix with insiders (ethnocentrism), avoiding outsiders (xenophobia), excluding any individuals that show signs of infection (shunning) or punishing those that behave in ways that may threaten others with disease, by displaying poor hygiene, for example. So as not to be punished or excluded, individuals self-police their own hygiene and social contact behaviour, sometimes turning disgust on themselves (shame). Group norms of hygiene behaviour (manners) may emerge and groups may agree to cooperate on activities that protect the group as a whole (public health). Because disgust is ‘strong magic’ that recognizes an ability to contaminate by association, it is used to marginalize outsiders to groups (stigmatization) and is employed in ritual and religion to demarcate what is pure and what is polluted. There is some evidence that disgust plays a role in morality, as much anti-social behaviour, as a form of social parasitism, is met with disgust. The workings of disgust as an adaptive system for disease avoidance in social groups have been discussed at length in a recent paper [7]. Here, I am concerned with the practical implications.

          There is much evidence that humans tend to shun other individuals that display signs of disease, as do ants, fish [90,91], bullfrogs [92], mice [93], lobsters [94] and chimps [95]. Human faces made up to look sick are found to be more disgusting than healthy counterparts [2]. Individuals perceived to have disabilities or disfigurements automatically activate disease-relevant cognitions, even when perceivers are explicitly aware that these individuals do not harbour contagious diseases [96,97]. A hypervigilant disgust may be triggered implicitly by a range of conditions that may, or may not, be associated with risk of infection, such as epilepsy, mental illness, mental retardation, obesity, skin conditions such as psoriasis, cancer and HIV [98]. People who are more concerned with disease are less likely to have friends with disabilities [99], to dislike obese individuals more [75] and to display implicit ageism [100]. Having a psychology that is hypervigilant to cues as to who might be carrying an infectious illness means that we are particularly sensitive to socially acquired information about who is sick. Power-seeking individuals can exploit this fact. A common tactic for the playground bully, for example, is to label another child as infected or as having ‘cooties’ the victim then suffers shunning by their peer group.

          Damaging as this can be to the individuals who are the subject of suspicion, stigmatization extends the problem of the labelling of individuals as diseased to whole groups. Out-groups, already a subject of suspicion because they could be carrying novel infections to which the in-group has not previously been exposed [1], can be especially easily labelled as disease carriers. A body of work has recently emerged that links parasite stress to assortative sociality (reviewed by Fincher & Thornhill [101]). Cultural groups that have historically faced high rates of parasite stress tend to be more xenophobic, have stronger family ties, and have more languages, ethnic groups and religions. There are a number of possible explanations for why this may be the case and confounding factors cannot be ruled out. However, it is clear that, throughout history, in-groups have been able to bolster groupishness by labelling members of out-groups as polluting, dirty, unhygienic, disease-carriers, so justifying caste and class divisions, cruelty, exploitation, pogroms, ethnic cleansing, genocide and war [102]. Such problems persist globally because the old tricks still work. The powerful continue to exploit our inherent tendencies to cleave to the in-group in the face of a disease threat from the outside. Intercommunal violence and discussion of immigration hence peak at election times [103,104].

          Because access to social life is so fundamental to our species, we are predisposed to learn not to inflict our own infectious emanations on others. We learn ‘good manners’ early covering our mouths when we cough and respecting designated defaecation locations, for example [7]. Failures in this department lead to a feeling of shame. Shame also leads those with conditions that they perceive as possibly infecting and hence repulsive to others to sequester themselves. Acne can cause shame and poor self image [105], and fistula can lead sufferers to remove themselves from the society for fear of causing offence [106]. Incontinence sufferers feel humiliated, as one doctor recounted from his own experience:

          To lay in bed, and against all physical rules, and I may say psychological rules as well, and do what you normally do at the toilet was a humiliating experience of the helplessness patients feel when help with basic functions is needed. Why did I never question this part of caring when I worked as a doctor? For us, defaecation was only an abstract category in the patient's medical record [107].

          A common fear among terminally ill people is that of losing control over their physical functions. Isaksen [108] suggests that this fear is based on becoming ‘dirty’ and hence ‘untouchable’ because of the fears that bodily fluids evoke in others. While the old, the frail, the sick and the disabled, who have to hand their body care to others, fear the disgust that they may occasion, overcoming revulsion of body products is one of the issues faced by carers. When the carer is a partner, this can put an extreme stress on the relationship [109] and is part of the, often unrecognized, emotional cost of caring [110].

          Like the sick, carers face a double whammy, in having not just to deal with the products of sickness but with social stigmatization. Individuals whose work involves contact with body products, hair, feet, sewage, used clothes, wastes and dead bodies tend to be poorly rewarded and suffer low status, perhaps because the nature of the work is perceived to contaminate the individual. Though common throughout the world, it is in the Hindu caste system where such occupational pollution is most visible—and damaging—despite recurrent efforts at reform [111]. Those that campaign against abortion, homosexuality and genetically modified foods exploit the imagery and language of disgust and its ability to contaminate they employ pictures of aborted foetuses, talk of ‘dirty’ sexual practices and raise the spectre of ‘Frankenfoods’. By labelling the outsider as dirty and diseased, racists and nationalists find that they can also, to some extent, recruit morality to their side [112]. The best defence against such manipulative tactics is first, to understand what is happening, and second, to expose such strategies to the light of public revulsion.

          Although disgust plays a key role in protecting us from disease, it is also responsible for much human suffering. Our evolved psychological defences against parasites are a double-edged sword. On the one hand, they provide the first line of defence against infection in social interaction. But at the same time they prevent social interaction, often at a time when it is most needed. Individuals who are sick or who have become contaminated by association, real or imagined, find themselves the subject of involuntary disgust reactions from others, facing disdain, suspicion and sometimes exclusion. Unscrupulous individuals make political capital from blaming and stigmatizing victims and the groups to which they belong, and the victims often turn blame and disgust on themselves.

          What can be done to prevent or reverse this unhappy cycle? The recent story of the response to the HIV pandemic holds lessons that give some cause for optimism. First, irrational fears of contamination were, in early days, recognized as a factor in the social response to the disease and the public was educated that victims were not contagious and did not pose a threat to the general population [113]. Groups that were particularly affected, such as homosexuals and sex workers, recognized that a process of stigmatization was underway and organized attempts to combat it. They refused collective stigma by declaring their individuality, for example through artistic productions such as plays, films, literature and events [114]. They supported one another to publicly refuse to accept shame and self-blame. Political activists, patients, academics and health professionals worked together to change public opinion about HIV and AIDS [115]. While the problem has not been fully solved—those living with HIV still suffer from stigma, exclusion and sometimes violence—the public debate and the political response did much to reduce the suffering of the affected and, beyond this, to raise general awareness of the social effects of infectious disease.

          5. Moral disgust

          There are a number of lines of argument that link disgust to our implicit sense of morality. Antisocial acts and individuals are often labelled as disgusting [1], similar physiological and brain activation has been observed in moral and biological disgust [9,116] and a number of studies have suggested that physiological disgust can affect moral judgement [117–119], though doubt has been cast on some of these findings [120]. While the link between disgust and morality needs further exploration, disgust clearly plays a major and visceral role in our response to wrongdoing, exploitation and injustice. We asked teenagers at one UK school to list what they found to be morally disgusting of several hundred examples the most common were rape, racism, killing, murder, torture, bullying, paedophilia, discrimination, necrophilia, genocide, exploitation, incest, theft, bestiality and cannibalism. Several authors have made a start on unpicking the nature of the relationship [1,6,121]—is it an exaptation of an ancient system designed to distance ourselves from parasites, turned to the function of ostracizing social parasites? Is it purely metaphorical? Or is disgust elicited because many of these offences involve bodily fluids? Whatever the explanation, there is no doubt that the emotion of disgust plays a major role in our decision-making about what is right and wrong. Nussbaum, for example, describes how the rhetoric of disgust influences judgement in the legal system. She argues that we should distrust our disgust responses, because they can lead to prejudice and discrimination [122]. While this may indeed be the case, Nussbaum fails to acknowledge that disgust also plays a major part in our ability to distinguish right and wrong—an ability which underpins our ability to function in ultrasocial groups. To outlaw disgust in moral judgement would be akin to throwing the baby out with the bathwater. As Leon Kass [123] has argued, there is ‘wisdom in repugnance’. Moral disgust is one of the major positive forces that builds, maintains and polices the cooperative societies in which we have to live to thrive. Understanding disgust's part in the morality puzzle remains a major task for social scientists—one that could offer important cues for the ways in which we make social policy.

          6. Conclusions

          Disgust is a powerful emotion that plays an under-appreciated part in all of our lives, not just in our everyday hygiene habits and in our manners, but in our response to disease, to social hierarchy, to those who are different from ourselves and to immorality. Disgust is a double-edged sword that is both the first line of defence against disease, but also a cause of much human suffering. Throughout this paper, I have argued that it is vital that we shine a spotlight into this less-explored darker recess of our psychology. Doing so allows us to enhance our abilities at disease prevention, to deal with many of our commonest anxieties and phobias better and to combat the many prejudices that plague human social relations. It may even help us to understand how to build more cooperative societies.

          Understanding disgust matters, both in itself, and for the practical consequences that stem from understanding. Understanding the function of the brain is possibly the most important intellectual challenge faced by scientists today. Emotions remain a contested subject in psychology with little agreement as to what they are, how they should be characterized, how they should be studied, even as to how many there are [124–126]. If disgust is an example of an emotion, then the new synthesis suggests that other emotions should have similar features. They should all have an adaptive purpose, an ancestral animal history, a set of cues that engage emotive processes and a set of typical behaviours, which may or may not be adaptive in the current environment [124]. Though they may have associated feelings, this is not definitive of an emotion. Disgust provides a rich test bed and a prototype for emotion studies.

          On the practical side, we have seen that understanding disgust has many benefits. Understanding disgust as a disease-avoidance mechanism can help us to change the behaviours that cause infection and chronic disease. Understanding how disgust tends to err on the ‘safe side’ helps to explain why exclusion of the sick, the old, the lower caste and the different is still so common and shows us that social movements can refuse such prejudice—even labelling it disgusting—as witnessed by the fact that the kids of today now find racism and homophobia disgusting. Understanding how individuals exploit disgust's ability to contaminate and spoil in their own quest for power can help us to expose such strategies.

          If understanding is the key to action, then yet more understanding is needed. The new synthesis that disgust is an adaptive system for infectious disease control has yet to be fully taken on board in the brain and behavioural sciences. Evolutionary theory offers a new means of integration of studies of brain architecture, neurochemistry and pathology, and of individual, social and cultural behaviour. The theory base for such work is vital—it provides both conceptual unity for prediction and a means for interpreting results. Theory should also inform the tools of such work—if psychological constructs are incorrectly conceived of, then the tools that are used to try to measure them will fail, giving false or misleading results. A key priority is the development of better measures of disgust. An early task for such tools is to use them to explore the relationship between subtypes of disgust and pathological phobias and anxieties.

          Further work is needed to elucidate the developmental pathways of disgust and to explore how predispositions interact with social norms to create manners, the first line of defence against interpersonal infection and a possible evolutionary precursor of morality. The origins of the human ability to live in ultrasocial groups is one of the hottest topics in evolutionary biology today [127–129]. Disgust provides a key thread that may help us to unravel this problem.

          Disgust studies, however, face the same problem of repulsion that they confront. Olatunji searched the published literature and found 10–20 times more papers per year on anger and fear than on disgust [67], perhaps owing to its lack of attractiveness when compared with other emotions. A similar problem afflicts public health. Though diarrhoeal diseases are the number two killer of children in the world today, they still attract only a fraction of the research funds that go to malaria or HIV, for example [16,130]. Lack of sanitation and hygiene are among the biggest culprits, yet it is hard to attract students to carry out studies of the faecal–oral transmission of infection, or on how to meet 40 per cent of the planet's urgent need for sanitation. A recent review found that menstrual hygiene had been comprehensively ignored in health research. Our group is devoting efforts to finding ways of making sanitation sexy, setting-up events such as the ‘Golden Poo Awards’ for example. Artistic exploration of the disgusting such as the Wellcome Trust's season on ‘Dirt’ and the ‘Grossology’ exhibitions that tour the world, help to attract interest and expose disgust to the light of day. Such efforts are beginning to pay off in terms of increased research funding.

          Disgust is a voice in our heads, it is the voice of our ancestors telling us to avoid infectious disease and social parasites. The voice of emotion is there for a reason, it guides us to behave in ways that are good for our genes, or more precisely, to behave in ways that were good for the genes of our ancestors. But we no longer live in the environments in which we evolved, and emotion is not the only voice in our heads. We have also evolved an executive brain which can listen to reasoned argument, weigh outcomes, learn from experience in new environments, and from science, and which can override emotional responses when the long-term benefits may outweigh the short-term gain [124]. Disgust is a vital force in our lives, we need to listen to it, to act on it and sometimes we need to over-ride it. Above all we need to understand it.

          Drug treatment can cure amebiasis within a few weeks. However, because medication cannot keep you from getting infected again, repeat episodes of amebiasis may occur if you continue to live in or travel to areas where amoebas are found. Among children in developing countries, especially infants and those younger than 5, gastrointestinal amebiasis can be fatal. Worldwide, amebiasis is the third most common cause of death from parasitic infections.

          External resources

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