What temperature counts as a fever?

The clinical threshold for fever is 100.4 degrees Fahrenheit, or 38 Celsius, measured properly. The famous 98.6 is a myth of false precision: it came from a 19th-century average, and modern research puts typical normal a bit lower and, more importantly, a range that swings by nearly a degree across the day, lowest in early morning, highest in late afternoon, and varies by person, age, and where you measure. A reading of 99.5 in the evening is often nothing.

A fever is the immune system deliberately raising the thermostat to fight infection, which is why doctors increasingly say to treat the person, not the number: if a mild fever comes with feeling basically okay, letting it run is reasonable, and medication is for comfort. The numbers that change the plan are the danger signs: any fever in an infant under three months is an emergency, and at any age a fever above about 103F, one lasting more than a few days, or one paired with stiff neck, confusion, trouble breathing, rash, or severe pain warrants prompt medical care rather than waiting it out.