There are some "selfish" genes which tell you to stay in bed whenever you feel sick, your nose is stuffed and your headache is spreading to your toes -- only to save others from infections, say researchers.
Along with the symptoms, the sick individual can become depressed and lose interest in social and sexual contact, limiting opportunities to transmit pathogens, they noted.
According to them, feeling sick is an evolutionary adaptation and evolution is functioning on the level of the "selfish gene".
Even though the individual organism may not survive the illness, isolating itself from its social environment will reduce the overall rate of infection in the group.
"From the point of view of the individual, this behaviour may seem overly altruistic. But from the perspective of the gene, its odds of being passed down are improved," said lead researcher Keren Shakhar from Weizmann Institute of Science in Israel.
The scientists went through a list of common symptoms, and each seems to support their hypothesis.
Appetite loss, for example, hinders the disease from spreading by communal food or water resources.
Fatigue and weakness can lessen the mobility of the infected individual, reducing the radius of possible infection.
"We know that isolation is the most efficient way to stop a transmissible disease from spreading," added Guy Shakhar from Weizmann Institute's Immunology Department in Israel.
"The problem is that today, for example, with flu, many do not realise how deadly it can be. So they go against their natural instincts, take a pill to reduce pain and fever and go to work, where the chance of infecting others is much higher," Shakhar concluded.
The study was published recently in the journal of PLoS Biology.
