According to evolutionary medicine, much illness is not directly caused by an infection or body dysfunction but is instead a response created by the body. Fever, for example, is not caused directly by bacteria or viruses but by the body raising its normal human body temperature, which some people believe inhibits the growth of the infectious organism. Evolutionary medicine calls this set of responses sickness behavior. These include such illness defining health changes as lethargy, depression, anorexia, sleepiness, hyperalgesia, and the inability to concentrate. These together with fever are caused by the brain through its top down control upon the body. They are, therefore, not necessary, and often do not accompany an infection (such as the lack of fever during malnutrition or late pregnancy) when they have a cost that outweighs their benefit. In humans, an important factor are beliefs that influence whether the health management system in the brain that evaluates costs and benefits deploys them or not. The health management system, when it factors in false information, has been suggested to underlie the placebo reduction of illness.
Read more about this topic: Illness
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