Fever is a core temperature (rectal) of 37.5°C–38.3°C (99.5°F–100.9°F), a skin temperature (axillary) >37.2°C (>99°F), a morning oral temperature >37.2°C (>99°F), or late afternoon oral temperature >37.7°C (>99.9°F), with lower thresholds applicable to frail elderly persons. 1