How and why does the body create fever?

Fever happens when the immune system senses a threat such as infection, and pumps out chemicals called cytokines. They, in turn, set in motion a series of chemical reactions that turn up the body's thermostat.

All of this takes place in a tiny spot in the brain's hypothalamus called the median preoptic nucleus, according to new research on mice by Dr. Clifford Saper, a neurologist, and his team at Beth Israel Deaconess Medical Center.

The goal of a fever is to raise body temperature - temporarily - to prompt infection-fighting white blood cells to fight harder. Even just a few degrees over the normal 98.6 degrees Fahrenheit makes a difference.

Some people believe that only bacterial, not viral, infections cause fevers, but both can, so you can't use fever to figure out which kind of infection you have, said Dr. Nesli Basgoz, associate chief for clinical affairs in the division of infectious diseases at Massachusetts General Hospital.

Fever can occur in response to problems other than infection, including such problems as drug allergies, autoimmune diseases, some hormonal conditions, and heat stroke, said Basgoz.

Doctors still debate whether treating a fever - with aspirin, Tylenol, or ibuprofen - significantly impedes the body's efforts to fight infection. "Even if it does make an infection last an extra day or so, it's reasonable to use medications to feel better," said Dr. Michael Shannon, a pediatrician and pharmacologist at Children's Hospital.

Fever in infants under 3 months old always needs medical attention, he said. For the rest of us, even fevers of 105 probably aren't dangerous unless accompanied by vomiting, difficulty breathing, unusual drowsiness, or severe headache.

E-mail health questions to foreman@globe.com.