- Fever defined as oral temperature greater than or equal to 38 degrees C (100.4 degrees F); rectal or core temperature greater than or equal to 38.6 degrees C (101.4 degrees F), or axillary temperature greater than or equal to 37.5 degrees Celsius (99.5 degrees F). Hypothermia defined as rectal or core temperature less than 36.0 degrees C (96.8 degrees F).