Why does body Temperature rise during Fever?
We have three groups of neurons in the hypothalamus, at the base of the brain, that are responsible for regulating body temperature: a hypothalamic thermostat that monitors skin temperature {via incoming signals from nerve fibers) and blood temperature; a heat-promoting center that raises body temperature; and a heat-losing center that lowers body temperature.
An infection stimulates the immune system to produce chemicals called endogenous pyrogens. These and prostaglandin E2 raise the set point of the hypothalamic thermostat {like turning up the thermostat in your house). The heat-promoting center constricts blood vessels near the skin surface so more heat is retained in the body, and if that's not enough, it stimulates shivering to generate more heat, until the body temperature reaches the new set point. The elevated temperature makes it harder for bacteria and viruses to multiply, so the immune system can overwhelm these pathogens faster than they reproduce and rid the body of the infection. Fever is thus a healthy mechanism that enables the body to recover from infection more quickly.