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If you inject a grasshopper with certain bacteria, it will jump to the sunniest blade of grass within reach and commence to develop a fever. Of course, if it had the option of putting its head under the covers, it might do that instead in the hope of thwarting infection.

Fever, it turns out, is one of life's oldest adaptations, an evolutionary "solution" that has been carefully passed from one phylogenetic family to another, like the gift of fire.

Fever is one of the oldest and most important signs in clinical medicine. Nonetheless, Americans spend at least $1-billion a year to get rid of it, even though very little is known about its effect on the course of disease. Moreover, the workings and purpose of fever are not fully understood by modern science.

A team of medical researchers in Baltimore last week reported that in human beings, fever from now on should be considered as any temperature higher than 98.9 Fahrenheit in the morning, or 99.9 later in the day. The long-established "normal" standard of 98.6 is not a sacred barrier beyond which lies evidence of illness.

Those findings reinforce the fact that the regulation of temperature is one of life's subtlest, and most mysterious, mechanisms.

Temperature not only helps determine behavior, fertility and metabolism; it also significantly affects the immune system - every animal's built-in medicine cabinet. Temperature, in many ways, is the key to the cabinet.

With such potent effects, it is of little surprise that temperature is both jealously defended and constantly adjusted in organisms virtually throughout the animal kingdom.

A part of the brain called the hypothalamus is the seat of temperature regulation in higher animals. Highly specialized cells there are sensitive to changes in the temperature of circulating blood, and also receive information from nerve cells in the skin and other remote locations. For human beings, the goal is to maintain temperature roughly between 98 and 99 F.

When temperature rises or falls significantly, an animal is driven to undertake behaviors that will adjust its temperature to the desired range. For people, this can be as simple as putting on or taking off a sweater. If such gross adjustments are not sufficient, the body will rely on internal devices, notably sweating or shivering.

When microorganisms invade the body, the immune system is stimulated to produce chemical substances, called "pyrogens," that change the sensitivity of the hypothalamic nerve cells. This ultimately creates a new and higher temperature "set point" for the system, one the organism seeks with the same fervor it previously showed for the lower one.

Cold-blooded animals work just as hard as warm-blooded ones to mount fevers, even though cold-blooded creatures have only behavioral adaptations to assist them. Infected goldfish, for example, will swim to the warmest part of a pond.

Given all the research that has gone into fever's effects in other species, it is perhaps surprising that so little is known about its importance in human disease. No clinical trials have ever determined whether, on balance, people with infections do better or worse if fever is left to run its course.

In fact, fever is one of the few symptoms of disease that virtually everyone is comfortable treating without medical advice. This is true in part because the most popular over-the-counter painkillers - aspirin, acetaminophen and ibuprofen - all reduce fever. Without a prescription, it is virtually impossible to treat pain without also treating fever.

Doctors, for their part, generally are not taught to respect evolution's prescriptions if they think they can write a better one.

For example, one researcher has developed a complex method of curing the common cold - something the common fever would very likely do by itself. Jack Gwaltney, a physician and virologist at the University of Virginia, is about to publish results of a clinical trial in which people infected with cold viruses got over their illnesses more quickly when given a three-part regimen of drugs that included inserting one in the nose, inhaling another and swallowing a third. It's a treatment not available anywhere else in the animal kingdom.

"I just do not think the scientific evidence shows that in the common cold, treating symptoms (such as fever) does anything detrimental to your immune system," he said. "To make the point that everything your body does is beneficial is incorrect."