An elevated body temperature.

While the standard for normal body temperature is 98.6°F (37°C), normal body temperatures actually fluctuate within a range of one to two degrees, making it impossible to formulate a precise definition of fever based on a specific temperature. Children's temperatures are generally higher than those of adults and fluctuate more widely. They may vary depending on the time of day, the child's emotional state or level of physical activity, the amount of clothing worn, or the surrounding room temperature. In general, temperatures under 100°F (37.7°C) are considered "subfebrile" (i.e., not indicating fever). Rectal temperatures of up to 100.4°F (38°C) may be considered normal.

A part of the brain called the hypothalamus acts as the body's thermostat, keeping its temperature at or close to 98.6°F (37°C). When there is an infection (or certain other types of disease), the body's white blood cells produce specific...