People can recognize subtle differences in the appearance of the skin, mouth and eyes as signs of disease.
John Axelsson at Stockholm University and his colleagues investigated whether people are able to detect illness in others in the absence of behaviours associated with infection, such as coughing. They showed untrained volunteers photographs of men and women taken two hours after injection with either a placebo or a bacterial molecule that triggers a strong immune reaction.
People who saw these pictures for no more than five seconds classified faces as either sick or healthy more accurately than if they had chosen at random. Pale skin and drooping eyelids were the strongest predictors of sickness.
The ability to detect early signs of disease might have evolved as a way to avoid close interaction when the risk of contagion is high.