I read that this is a common misconception: the high heat is not enough to denature any proteins (else it would kill you too) and, what’s more surprising, it actually makes viruses/bacteria more active. But it also makes your immune system more active, with an overall win in effectiveness over the microbes, which is what makes it useful.
Yep - our bodies turn the thermostat up, increasing metabolism/cellular functions, which increases body temperature. Fatigue slows us down as our bodies redirect resources towards supporting our immune systems and producing cells to fight off the infection, vs spending that energy on being mentally and physically active.
Once our bodies get a handle on things, the fever “breaks” and we start recovery and return to homeostasis.