Feelings of disgust help humans avoid, or at the very least recognize, the things that cause disgust like sick people, dirty water, vomit, body fluids and all the other stuff that makes us react "Yuck."

BBC News reports that 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 behavior is essential to prevent the spread of all the major current and recent infectious diseases which present a threat to humans.

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, feces 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 defense against disease.

Something as simple as handwashing 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.

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."

Why disgust matters
12.dec.11
Philosophical Transactions of the Royal Society B, vol. 366, no. 1583, 3478-3490
Valerie Curtis
http://rstb.royalsocietypublishing.org/content/366/1583/3478
Abstract
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.