Copying Google AI's response here as it's at least as good as what I was going to recall:

> Fever is a key part of the innate immune system, acting as a protective response to infection by raising the body's temperature. This increase in temperature inhibits the growth of many pathogens, enhances the activity of immune cells like leukocytes, and improves the effectiveness of the adaptive immune response.

My Vietnamese in-laws commonly make a sweat tent to shorten the duration of sickness. I can't say if it works, but it's something I intend to try next time I'm sick.