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.
