Fever. In most adults, a fever starts at an oral or axillary temperature of 37.6°C (99.7°F) or a rectal or ear temperature of 38.1°C (100.6°F). A child has a fever when his or her rectal temperature is 38°C (100.4°F) or higher or armpit (axillary) temperature is 37.6°C (99.7°F) or higher. ... This is called a "drug fever."