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Fever is a frequent medical symptom that describes an increase in internal body temperature to levels that are above normal (37°C, 98.6°F). Fever is most accurately characterized as a temporary elevation in the body’s thermoregulatory set-point, which is usually by about 1-2°C. Fever differs from hyperthermia, which is an increase in body temperature over the body’s thermoregulatory set-point .

The elevation in thermoregulatory set-point means that the previous "normal body temperature" is considered hypothermic, and effector mechanisms kick in. The person who is developing the fever has a cold sensation, and an increase in heart rate, muscle tone and shivering attempt to counteract the perceived hypothermia, thereby reaching the new thermoregulatory set-point.