Many of the germs that cause disease have a very specific temperature range. If the host is too hot or too cold, the germs don't thrive. When you catch the flu, your body tries to make things uncomfortable for the invaders. Your hypothalamus releases chemicals that cause your muscles to tense up, producing more heat. At the same time, the blood vessels near your skin constrict. This keeps your body from losing the extra heat it is generating, and also makes you feel cold, so you bundle up even more. If that does not do the trick, you will get "chills", making you shiver, which generates even more heat.

Once your body reaches the temperature that it thinks should kill off the germs, you feel hot. You kick off the covers to cool off. If you cool off too much, the chills come back, to keep your temperature high enough to keep the germs from settling in for a long stay.

In addition to making things hot for the germs, a fever also stimulates your immune system, making it more effective.

Of course, when this happens, we grab for some medication to get rid of the fever. This makes us feel better, but studies have shown that lowering your fever can actually make the disease last longer. Of course, high fevers can be dangerous, but a low grade fever can actually be a good thing.