Not very long ago -- when my parents were children -- a high fever in a child could easily mean the end for that child. High fevers resulting in death were common. Today, most of the devastating illnesses causing these high fevers are either treatable (thanks largely to antibiotics), or better yet preventable (thanks largely to vaccines). Nevertheless, for generation after generation on our planet, a high fever in a child was a chilling terror to the parents -- their dream, their darling might slip from them despite everything they could do. Fever as enemy is deeply ingrained in our cultural memory.

I was surprised to learn in medical school that fever, far from being an enemy, is an important part of the body's defense against infection. While a fever in a child signals to us that a battle might be going on in a child's body, the fever is fighting for the child, not against.

Most bacteria and viruses that cause infections in humans thrive best at 98.6 degrees F (37C). Raising the temperature a few degrees can give the body the winning edge. In addition, a fever activates the body's immune system, accelerating the production of white blood cells, antibodies, and many other infection-fighting agents.

Cold blooded animals adjust their temperatures by moving to a different spot. While a medical student, I sat fascinated as I learned that lizards, when infected, will move to a warmer place to give themselves a fever. If they are prevented from finding a warmer spot, they will likely die.

Many parents fear that fevers will cause brain damage. Brain damage from a fever will not occur unless the fever is over 107.6 degrees F (42C) for an extended period of time. Many also fear that untreated fevers will keep going higher and higher, up to 107 degrees F or even more. Untreated fevers caused by infection will seldom go over 105 degrees unless the child is overdressed or trapped in a hot place. The brain's thermostat will stop the fever from climbing above 106 degrees F. Some parents fear that fevers will cause seizures. For the great majority of children this is not the case. About 4% of children, though, will sometimes have seizures with fever. These febrile seizures are caused by rapid increase in temperature, not by the height of the temperature. Once a child already has a high fever, a febrile seizure is unlikely with the current illness. In any event, febrile seizures are over in moments with no lasting consequences. Treating fevers early in these children may prevent further febrile seizures. Antiseizure medications are only considered if the child has frequent febrile seizures – especially if they do not respond to acetaminophen or ibuprofen (Pediatrics in Review, November 2007).

So, what temperature constitutes a fever?

While 98.6 degrees F is considered the normal core body temperature, this value varies between individuals and throughout the day. The daily variation is minimal in children less than six months of age, about 1 degree in children 6 months to 2 years old, and gradually increases to 2 degrees per day by age six. A person's baseline temperature is usually highest in the evening. Body temperature, especially in children, is normally raised by physical activity, strong emotion, eating, heavy clothing, elevated room temperature, and elevated humidity.

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