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.read more >>