Fever, the regulation of body temperature at an elevated level, is a common response to infection throughout the vertebrates. Mammals and birds rely on both physiologic and behavioral mechanisms to raise their body temperatures to this elevated thermoregulatory "set-point" during infection. Lower vertebrates such as fishes and reptiles primarily rely on behavior to elevate their body temperatures. For example, the febrile lizard will spend greater lengths of time near a heat source, and as a result its body temperature rises. A fever appears to be induced by a variety of substances such as bacteria, viruses, and fungi. These inducers of fever result in various types of phagocytes producing a heat-labile protein(s?), endogenous pyrogen. It is this endogenous pyrogen that is thought to result, ultimately, in the thermoregulatory set-point being raised. Within the past several years considerable evidence has accumulated that moderate elevations in body temperature are beneficial to the infected host. Studies with bacterial and viral infected animals have shown that moderate fevers increase survival rate. Many components of the nonspecific host defense response to infection such as leukocyte mobility, lymphocyte transformation, and effects of interferon, appear to be enhanced by elevations in temperature that simulate moderate fevers. In addition, some evidence indicates that a fever in conjunction with the changes in plasma iron levels known to occur during infection is a synergistic host defense response. More research needs to be done to determine for specific diseases whether moderate fevers are beneficial, neutral, or harmful to the infected host.