Thermoregulation: why you shiver during a fever

You have a 39 °C fever, yet you pull on a blanket and shiver like it is winter. That feels backwards. Surely a hot body should want to cool down, not warm up further?

It is not backwards. Chemicals called pyrogens, made by pathogens and blood cells during infection, push the hypothalamus's set point up from 37 °C to something higher, say 39 °C. Your body is still near 37 °C, so it is now cold relative to that new target, and it shivers to catch up.

Fever is not the thermostat breaking. It is the same control system defending a deliberately higher set point.