a fever is when a person's body temperature is hotter than 37.5 degrees celsius (99.5 fahrenheit). normal body temperature for humans varies based on a variety of factors, including age and level of physical activity. it is typically cited as 37 degrees celsius (98.6 f), but naturally varies from person to person by at least .5 degrees celsius. the actual [[measurement] of body [[temperature]] will vary based on the location of the [[measurement]]. for a [[temperature]] taken from under the [[tongue]], the measurement may be lower. [[rectal]] temperatures will read about 0.3 c (0.55 f) higher, and armpit temperatures will read about the same amount lower.