The chills, the sweats...fevers not only feel terrible, they also make the body work hard! For every two degrees Fahrenheit your temperature goes up, your heart rate increases ten percent. Why does the body go through such laborious misery when we're sick?

The good news is that there is a method to fever's madness. Fever helps the body fight infection by revving up the immune system. Turn up the heat in the body, and many body processes speed up, including the production of white blood cells and how fast they find infectious bacteria and viruses. Once there, white blood cells called macrophages gobble up infectious organisms. Although higher body temperatures might kill some bacteria or viruses, fevers help us most by boosting the body's own immune response.

Fevers are such valuable responses to infection that creatures all along the evolutionary scale get them. That includes friendly neighborhood dogs and cats, as well as critters you might not expect, like insects and lizards. In a lab, after a lizard is injected with live bacteria, it will creep under a heat lamp in order to raise its body temperature. That's not so different from piling on the blankets when you've got the feverish chills!

Because fever revs up our immune response, reducing fever with medication could make the infection last an extra day. But when you're feeling miserable, the tradeoff's probably worth it!