Fever is a condition in which the brain maintains the body temperature at a higher than normal level. It is one of the most common symptoms of disease. When fever is the main symptom of a disease, it may be part of the disease’s name, as in scarlet fever or yellow fever.
Not every rise in body temperature is a fever. For example, sitting in a sauna can produce an above-normal body temperature. But in this case, unlike what happens in a fever, the brain instructs the body to lower its temperature by sweating and increasing skin blood flow, and the individual feels the urge to be in a cool place.
Fever results when an infection or an allergic or toxic reaction causes the brain’s temperature setting to rise. For example, after a flu virus enters the human body, the body releases proteins called endogenous pyrogens or leukocyte pyrogens. These proteins trigger the release of certain chemicals in the hypothalamus, the part of the brain mainly responsible for regulating body temperature. The chemicals, called prostaglandins, act on nerve cells to produce a sensation of coldness. This causes the hypothalamus to raise body temperature by making the body burn fat, decrease skin blood flow, shiver, and develop an urge to stay warm. Antipyretic drugs, such as aspirin and acetaminophen, reduce fever by slowing prostaglandin production.
Medical research has shown that fevers speed up the body’s defenses against invading viruses and bacteria. Because fever thus can help fight infection, some medical experts advise against reducing a moderate fever. In human beings, normal body temperature ranges from about 98 °F (36.7 °C) to 99 °F (37.2 °C), depending on the individual and the time of day. A moderate fever generally ranges from 100 °F (37.7 °C) to 102 °F (38.9 °C). Most experts agree that fevers probably should be reduced if they rise above 102 °F (38.9 °C), or if they occur in pregnant women, people with heart disease, or the elderly. When deciding how to handle a fever, it is wise to consult a doctor.
Fever occurs in all vertebrates (animals with backbones) and in many invertebrates (animals without backbones). Scientists think fever first developed in animals at least 300 million years ago as a way to fight disease. In warm-blooded vertebrates—that is, birds and mammals—fever results mainly from bodily reactions. Cold-blooded vertebrates, such as fish and reptiles—as well as such invertebrates as grasshoppers and scorpions—achieve fevers by moving into the heat, where they can maintain a high body temperature.
