Peter Nalin, associate professor of clinical family medicine and director of the family practice residency program at Indiana University, explains.

Fever is an elevated temperature of the human body that is substantially beyond the normal range. Normal body temperature fluctuates daily from about one degree below 98.6 degrees Fahrenheit to one degree above that number. Lower body temperatures usually occur before dawn; higher temperatures in the afternoon.

Body temperature also varies slightly depending on where on the human body it is measured. Rectal (internal) temperature tends normally to be higher than skin (surface) temperature. Oral and armpit temperatures can approximate actual body temperature and are more convenient to measure.

The presence of a fever is usually related to stimulation of the body's immune response. Fever can support the immune system's attempt to gain advantage over infectious agents, such as viruses and bacteria, and it makes the body less favorable as a host for replicating viruses and bacteria, which are temperature sensitive. Infectious agents are not the only causes of fever, however. Amphetamine abuse and alcohol withdrawal can both elicit high temperatures, for example. And environmental fevers--such as those associated with heat stroke and related illnesses--can also occur.

The hypothalamus, which sits at the base of the brain, acts as the body's thermostat. It is triggered by floating biochemical substances called pyrogens, which flow from sites where the immune system has identified potential trouble to the hypothalamus via the bloodstream. Some pyrogens are produced by body tissue; many pathogens also produce pyrogens. When the hypothalamus detects them, it tells the body to generate and retain more heat, thus producing a fever. Children typically get higher and quicker fevers, reflecting the effects of the pyrogens upon an inexperienced immune system.

Should one eat little or nothing while feverish, as the saying "Feed a cold, starve a fever" suggests? Yes. The reasons for this are threefold. First, during fever, all the body's functions are occurring amidst increased physiologic stress. Provoking digestion during physiologic stress over stimulates the parasympathetic nervous system when the sympathetic nervous system is already active. Second, it is possible that the body could misinterpret some substances absorbed from the gut as allergens during a fever. Finally, excessive fever can, on rare occasions, cause seizures, collapse and delirium--all of which may be further complicated by recent eating.

Fever can help fight infection, but sometimes it can climb too high for the body's own good. Internal body temperatures in excess of 105 degrees F, for instance, expose proteins and body fats to direct temperature stressors. This form of heat distress can threaten the integrity and function of proteins accustomed to the body's usual temperature variations and the occasional less excessive fevers. Cellular stress, infarctions, necrosis, seizures and delirium are among the potential consequences of prolonged, severe fevers. The receptor environment at the hypothalamus maintains limitations on high fevers. In the rare instances in which the hypothalamus itself malfunctions, the result is typically low body temperature, not elevated body temperature.