Fever happens when the body's internal "thermostat" raises the body temperature above its normal level. This thermostat is found in a part of the brain called the hypothalamus. The hypothalamus knows what temperature your body should be (usually around 98.6°F/37°C) and will send messages to your body to keep it that way. Most people's body temperatures even change a little bit during the course of the day: It's usually a little lower in the morning and a little higher in the evening. In kids, it can vary as they run around, play, and exercise. Sometimes, though, the hypothalamus will "reset" the body to a higher temperature in response to an infection, illness, or some other cause.
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