Why feeling disgust can be good for us
The facial expression for disgust is universal. We can all picture the contorted, horrified face which communicates a feeling of revulsion and loathing.
Spiders, slimy creatures, mucus and faeces can all provoke this feeling. Our reaction is to distance ourselves from the cause.
As a result, feelings of disgust help us to avoid, or at the very least recognise, the things that make us feel this way - and for a very good reason, psychologists say.
When it comes to infectious diseases, disgust has evolved to help us steer clear of sick people, dirty water, vomit, body fluids and all the other stuff that makes us react "Yuck."
In a paper published in Philosophical Transactions for the Royal Society B, Dr Val Curtis, from the London School of Hygiene and Tropical Medicine, argues that avoidance behaviour is essential to prevent the spread of all the major current and recent infectious diseases which present a threat to humans.
Diarrhoeal diseases, respiratory tract infections, malaria, measles, HIV, tuberculosis and parasitic worms can all be avoided by thinking about aspects of hygiene, Dr Curtis says.
Washing hands and food can prevent diseases like cholera and hepatitis A, avoiding sex with others who are infected helps prevent the spread of HIV, while keeping a distance from people with influenza or measles is a sensible move to reduce the risk of infection.
"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," writes Dr Curtis.
"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."
Something as simple as hand-washing with soap could save over a million lives a year globally, the paper says, just by stopping the transmission of disease.
Disgust is often used to get this message across in public health campaigns.
The UK government's information leaflet at the time of the H1N1 influenza outbreak in 2009 shows a human sneeze exploding directly towards the reader, spraying bodily secretions in all directions.
Its purpose was to encourage better hygiene and more sensible use of tissues.
The British Heart Foundation's most successful campaign showed the impact of smoking on the arteries by depicting cigarettes dripping globs of fat.
A recent study showed that the greater the disgust felt at such pictures, the greater the likelihood that people would have tried to, or succeeded in, stopping smoking.
So while disgust prevents us coming into contact with germs and viruses that might cause us to become ill, it can also feed our anxieties and lead to obsessive behaviour.
Graham Davey, professor of psychology at the University of Sussex, says disgust is not an innate emotion.
"We only develop an understanding of disgust around the age of two or three years old. Babies stick anything in their mouth before then and only learn when they see their mothers' facial expression signalling disgust at something they have done."
Disgust can also colour our view of something and make us act irrationally, particularly when it comes to phobias of small animals.
Spider phobias are very common and yet spiders in the UK are not harmful. Neither are rodents, creepy crawlies or insects, but they all trigger feelings of disgust and are often the cause of phobias.
"People tend to have phobias about animals which aren't necessarily harmful," says Prof Davey.
"It's different from fear. It is basically a disease-avoidance emotion - disgusting animals or objects tend to be vehicles carrying disease.
"But the spider is entirely innocent. It has a historic link with disgust and people tend to think they have venomous bites, which they rarely do, so there is not much rational grounds for that fear."
There is also evidence that disgust is involved in many mental health problems, including eating disorders, sexual dysfunctions, claustrophobia and psychosis.
Prof Davey said: "Some people have high sensitivity to disgust and that can be blown up into full-blown anxiety disorders.
"These can disrupt every part of a person's life, often getting in the way of work and relationships."
Stephen Fry, who has declared himself celibate in the past, is quoted in Dr Curtis's paper describing how disgust played a part in his decision to abstain from sex.
"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 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."