Symptoms of being sick are not an adaptation that works at the individual level, but are on the level of the “selfish gene,” proposed researchers from the Weizmann Institute of Science. Their findings are published in PLoS Biology

Feeling sick affects multiple body systems such as the immune system, endocrine system, and nervous system. Study authors explained that behaviors often associated with sickness seemed to have been “preserved over millennia of evolution.”

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Though the sick organism may not survive the illness, isolating itself from its social environment will reduce the overall rate of infection in the group. Dr. Keren Shakhar, of the Psychology Department of the College of Academic Studies, described this behavior as appearing altruistic from the individual’s perspective but “from the perspective of the gene, its odds of being passed down are improved.” 

The study includes a review of typical symptoms of illness and how they support the hypothesis. For example, appetite loss prevents the disease from spreading through shared food or water resources. Fatigue and weakness decreases the sick individual’s mobility thus reducing the radius of possible infection. Other symptoms such as depression or lost interest in social and sexual contact also reduce opportunities for pathogens to be transmitted. In addition, lack of personal grooming and changes in body language also project a “I’m sick! Don’t come near!” message.

Although isolation is the best way to prevent the spread of disease, people with the flu just take medication to reduce pain and fever and still go to work “where the chance of infecting others is much higher,” said Professor Guy Shakhar, from the Immunology Department. Study findings emphasize the importance of staying home if you feel sick. 

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