It was a warm June morning when the harried and hurried father strapped his 5-month-old son Bryan into the backseat of his car and headed for work. Tragically, the father forgot to drop his son off at the babysitter's. When his wife called him at work late that afternoon to inquire why the child was not at the sitter's, the shocked father realized his mistake and hurried down to his parked car. But it was too late--Bryan had died. Left for 10 hours in the car in the sun, all windows shut, the baby's temperature had quickly soared. Two hours after he was discovered, the child's temperature still exceeded 41°C (106°).

Sarah L.'s case of elevated body temperature was more typical. She awoke with a fever of 40°C (104°F) and a terribly painful sore throat. Peering down the five-year-old's throat with a flashlight, her mother spotted the telltale whitish lesions that usually indicate a Streptococcus infection. At the doctor's office, a test revealed that Sarah did indeed have a strep throat. The fever was her body's attempt to fight the infection.

The true cases of young Bryan and Sarah illustrate two reasons why body temperature may rise--inability of the temperature homeostatic mechanism to handle an extreme environment, and an immune system response to infection.

In the case of Bryan, sustained exposure to very high heat over-whelmed the temperature-regulating mechanism resulting in hyperthermia. Body heat built up faster than it could dissipate, and body temperature rose, even though the set point of the thermostat was normal. His blood vessels dilated so greatly in an attempt to dissipate the excess heat that after a few hours his circulatory system collapsed.

In a fever, molecules on the surfaces of the infectious agents (usually bacteria or viruses) stimulate phagocytes to release a substance called interleukin-1. It is also called endogenous pyrogen, meaning fire maker from within. The bloodstream carries IL-1 to the hypothalamus, where it raises the set point controlling temperature. In response, the brain signals skeletal muscles to increase heat production, blood flow to the skin to decrease, and sweat glands to decrease secretion. As a result, body temperature rises to the new set point, and the person has a fever. The increased body temperature helps the immune system kill the pathogens--and to make the patient quite uncomfortable for a while.

What should one do when body temperature rises? Hyperthermia in response to exposure to intense, sustained heat should be rapidly treated by administering liquids to replace lost body fluids and electrolytes, sponging the skin with water to increase cooling by evaporation, and covering the person with a refrigerated blanket. Some health professionals believe that a slightly elevated temperature should not be reduced (with medication or cold baths), because it may be part of a normal immune response.