Fever (also known as pyrexia, or a febrile response from the Latin word febris, meaning fever, and archaically known as ague) is a frequent medical symptom that describes an increase in internal body temperature to levels that are above normal (the common oral measurement of normal human body temperature is 36.8±0.7 °C or 98.2±1.3 °F). Fever is most accurately characterized as a temporary elevation in the body’s thermoregulatory set-point, usually by about 1–2°C. Fever differs from hyperthermia, which is an increase in body temperature over the body’s thermoregulatory set-point (due to excessive heat production or insufficient thermoregulation, or both). Carl Wunderlich discovered that fever is not a disease but a symptom of disease.

There are many variations in normal body temperature, and this needs to be considered when measuring fever.