A fever is an abnormally high body temperature. Normal body temperature is about 98.6° F. Persons have lived after having temperatures of 112° F., but only when the fever lasted a short time. The immediate cause of fever is the reduction of heat loss from the body as a result of constriction of the blood vessels accompanied by an increase in heat production. When the body is exposed to cold from outside, it also reacts in this way. However, a fever is stimulated by a disturbance, usually an infection, within the body. In the early stages of fever the skin is cold and pale; the person may have sensations of a chill, his teeth may chatter, and he may have gooseflesh.    The chills are caused by the reduced blood flow near the surface of the skin. The fall in skin temperature produces the fine contractions of muscles known as shivering. This raises the temperature still more. Finally, the temperature reaches a point at which the body reacts by allowing blood to flow to the surface of the skin once again. The person becomes flushed and feels hot. At this stage heat is lost from the body, and the temperature goes down. This process is helped by perspiring. In disease this cycle may be repeated several times. Fevers, when not too severe, are apparently useful in fighting infections. Since fevers require energy, it is usual to give a person plenty of food during recovery