You wake up at 6 am feeling achy and chilled. Unsure if you’re sick or just sleep-deprived, you reach for a thermometer. It beeps at 99°F, so you groan and roll out of bed to get ready for work. Because that’s not a fever. Is it? Yes, it is. Forget everything you thought you knew about normal body temperature and fever, starting with 98.6. That’s an antiquated number based on a flawed study from 1868 (yes, 150 years ago). The facts about fever are a lot more complicated.

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