’Animal behaviorists and neuroimmunologists use the term sickness behavior to describe the observable behavior changes that occur during illness.
Health care providers often treat these symptoms as little more than annoying side effects of having an infectious disease. But as it turns out, these changes may actually be part of how you fight off infection.
I’m an anthropologist interested in how illness and infection have shaped human evolution. My colleagues and I propose that all these aspects of being sick are features of an emotion that we call “lassitude.” And it’s an important part of how human beings work to recover from illness.
Your body sets priorities when fighting germs
The human immune system is a complex set of mechanisms that help you suppress and eliminate organisms – such as bacteria, viruses and parasitic worms – that cause infection.
Activating the immune system, however, costs your body a lot of energy. This presents a series of problems that your brain and body must solve to fight against infection most effectively. Where will this extra energy come from? What should you do to avoid additional infections or injuries that would increase the immune system’s energy requirements even more?
Fever is a critical part of the immune response to some infections, but the energy cost of raising your temperature is particularly high. Is there anything you can do to reduce this cost?
To eat or not to eat is a choice that affects your body’s fight against infection. On one hand, food ultimately provides energy to your body, and some foods even contain compounds that may help eliminate pathogens. But it also takes energy to digest food, which diverts resources from your all-out immune effort. Consuming food also increases your risk of acquiring additional pathogens. So what should you eat when you’re sick, and how much?
We humans are highly dependent on others to care for and support us when we’re sick. What should you do to make sure your friends and family care for you when you’re ill?
My colleagues and I propose that the distinctive changes that occur when you get sick help you solve these problems automatically.
Fatigue reduces your level of physical activity, which leaves more energy available for the immune system.
Increased susceptibility to nausea and pain makes you less likely to acquire an infection or injury that would further increase the immune system’s workload.
Increased sensitivity to cold motivates you to seek out things like warm clothing and heat sources that reduce the costs of keeping body temperature up.
Changes in appetite and food preferences push you to eat (or not eat) in a way that supports the fight against infection.
Feelings of sadness, depression and general wretchedness provide an honest signal to your friends and family that you need help.…’