Fever is defined as a body temperature above the normal range due to an increase in the temperature regulatory set-point. There is not a single agreed upon upper limit for normal temperature with sources using values between 37.5 and 38.3 °C (99.5 and 100.9 °F).
The increase in set-point triggers increased muscle tone and causes a feeling of cold resulting in greater heat production and efforts to conserve heat. This results in an increase in body temperature. When the set-point temperature returns to normal a person feels hot and may begin to sweat.