Pyrexia  is the medical term for fever, pyrexia is when the body temperature increase to above normal (37°C).

Temperature increases occur in response to invading microorganisms. When macrophages are exposed to bacteria and other foreign substances they secrete chemicals called pyrogens which act on the temperature regulating centre in the brain, causing the temperature to rise. High fevers can be dangerous, however mild to moderate fever can benefit the body by increasing repair processes, and reducing the accessibility of iron and zinc which is required by the bacteria to proliferate.

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More about Body Temperature

Humans are homoeothermic (endothermic). This means that they maintain a steady body temperature (like other mammals, and birds). Body temperature is controlled by the hypothalamus. Slight variations in body temperature cause the hypothalamus to send nerve impulses causing actions to either raise or lower temperature.

Effect of Temperature on Enzymes
Enzyme catalysed reactions are sensitive to changes in temperature. As temperature increases, there is an increase in the rate of reaction up to the level of perhaps 45-55°C. As the temperature raises higher, enzyme protein is damaged and in turn, the rate of the reaction decreases.