The next time you are stuck with a fever, stuffy nose, or down with the flu, you should probably let your “selfish genes” kick in.

Feelings of sickness are an evolutionary adaptation to keep the patient isolated and prevent transfer of disease to others, according to researchers Guy Shakhar of the Weizmann Institute of Science and colleague Dr. Keren Shakhar. The so-called “selfish genes” partly cause the symptoms, along with the bugs and germs that cause the condition in the first place.

This means the symptoms that make the patient so miserable are genetically designed to keep infection spread at bay.

The findings were published in the journal PLOS Biology.

How ‘Selfish Genes’ Work

Symptoms accompanying a certain illness seem to oppress the sufferer, but they do not simply work on the individual level. Instead, they function on the level of what is referred to as the “selfish gene," where social isolation operates to reduce overall infection rate in a specific group.

"From the point of view of the individual, this behavior may seem overly altruistic, but from the perspective of the gene, its odds of being passed down are improved,” said Dr.Keren Shakhar, an immunology professor.

The researchers combed through a range of common symptoms, each appearing to support their hypothesis. Weakness and fatigue reduce mobility of the infected and therefore the potential reach of infection, while appetite loss prevents communication of illness via food or water sources.

Even feelings of depression and losing social interest seem to agree with the hypothesis, limiting opportunities for contact and bacteria or virus transmission.

How about seeming changes in grooming, hygiene, and body language? They are a way of the sick to say: “I am sick, so stay away!”

Sickness behavior, according to the team, affects species from humans to arthropods and is orchestrated by an intricate web of network of cytokines and neuroendocrine pathways. “[C]learly, it has been naturally selected,” they stated.

Feeling Sick? Stay At Home

The problem, warned Shakhar, is that many people do not heed this evolutionary adaptation, going to work and proceeding with daily normal activities even while suffering the pangs of illness.

Shakhar cited the influenza virus as an example. “[Patients] 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,” he explained.

The authors added that the fact that the infection causes the symptoms is often taken for granted, with numerous bodily functions such as the immune, endocrine, and nervous systems involved in the process of sickness.

This evolutionary phenomenon is not limited to humans. Sick pets also act differently, while social insects such as bees exhibit extreme behavior by abandoning their dwelling to die somewhere else.

“In other words, such behavior seems to have been preserved over millennia of evolution,” the authors emphasized.

The main takeaway: heed common “sick symptoms,” stay at home, and you have a better chance of not passing the sickness on to others.

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