Cold & flu symptoms? Evolution’s telling you to stay home!

We tend to take it for granted that infection is what causes the symptoms of illness, assuming that the only significance of symptoms is to alert us to the fact that we are under the weather.

In truth, many of our body’s systems are involved in being sick: the immune system and endocrine systems, as well as our nervous system. Moreover, the behavior we associate with sickness is not limited to humans.

Anyone who has a pet knows that animals act differently when they are ill. Some of the most extreme “sickness behavior” is found in such social insects as bees, which typically abandon the hive to die elsewhere when they are sick. In other words, such behavior seems to have been preserved over millennia of evolution.

The selfish gene?

Why does this phenomenon persist? According to the scientists writing in  PLoS Biology symptoms have a meaning beyond the level of the individual. Rather, they suggest, evolution is functioning on the level of the “selfish gene” which ensures that even if the individual organism doesn’t survive the illness, isolating itself from its social environment (such as staying home when you have a cold) will reduce the overall rate of infection in the group.

“From the point of view of the individual, this behavior may seem overly altruistic,” says co-author Dr. Keren Shakhar, of the Psychology Department of the College of Management Academic Studies at the Weizmann Institute,”but from the perspective of the gene, its odds of being passed down are improved.”

Symptoms sending a message

In the paper, the scientists go through a list of common symptoms, and each seems to support the 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.

Along with the symptoms, the sick individual can become depressed and lose interest in social and sexual contact, again limiting opportunities to pass their illness on to others. Lapses in personal grooming and changes in body language say: I’m sick! Don’t come near!

“We know that isolation is the most efficient way to stop a transmissible disease from spreading,” says co-author Prof. Guy Shakhar of the Weizmann Institute’s Immunology Department “The problem is that today, for example, with flu, many do not realize 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.”

As cold and flu season is upon us the scientists say they hope its message sinks in: Millions of years of evolution are not wrong. When you feel sick, it’s a sign you need to stay home.