A stuffed nose, throbbing head and aching legs are your body's way of telling you to stay in bed, instead of crawling into the office, scientists say.
Feeling sick is an evolutionary adaptation according to a hypothesis put forward by Professor Guy Shakhar from the Weizmann Institute of Science and Dr Keren Shakhar of the College of Management Academic Studies in Israel.
We tend to take it for granted that infection is what causes the symptoms of illness, assuming that the microbial invasion directly impinges on our well-being, researchers said.
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 behaviour 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 behaviour" is found in such social insects as bees, which typically abandon the hive to die elsewhere when they are sick.
In other words, such behaviour seems to have been preserved over millennia of evolution, said researchers.
Symptoms are not an adaptation that works on the level of the individual. Rather evolution is functioning on the level of the "selfish gene," scientists say.
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 Keren.
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 transmit pathogens.
"We know that isolation is the most efficient way to stop a transmissible disease from spreading," said Guy.
"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," said Guy.
The study was published in the journal PLoS Biology.
