Our last program explored how a fever revs up the immune system to fight off infection. Turn up the heat in the body, and that speeds up many body processes, including our immune response. Today we'll find out exactly what's happening in the body when you get feverish chills and sweats--other than sheer misery!

When immune cells detect an infectious organism, they produce fever-making chemicals. These chemicals prompt the body to produce prostaglandins, which turn up the thermostat in the brain. Let's say prostaglandins reset the thermostat to 102 degrees. Your body works hard to raise its temperature, but in the meantime, you get the chills. Why? Whenever the thermostat in the brain is set above your actual body temperature, you feel chilly. As soon as your body temperature rises to match the thermostat at 102, the chills will stop.

Just the opposite occurs as a fever breaks: the thermostat in the brain is turned back down to normal, but it takes time for the body to release the excess heat. You'll feel really hot and sweaty until your temperature falls to equal that of the thermostat. The biggest sweats usually come at the end of a fever, and the biggest chills at the beginning. But minor temperature changes throughout the fever cause more subtle chills and sweats.

Although fevers feel awful, the fever itself won't do any harm, unless it goes above 106 degrees. But if a fever's accompanied by a severe headache, extreme sleepiness, dehydration, or difficulty breathing, then it's time to see the doctor.