Fever

Regulation of Body Temperature

Normal body temperature is 37 °C (98.6 °F) when taken orally. However, variability exists between people, and temperature may fluctuate depending on the time of day, weather, exercise, menstrual cycle in women, and age; therefore, an individual's baseline temperature may deviate slightly from 37 °C. For about 99% of the population, normal oral temperature ranges from 36.0 to 37.7 °C (96.8–99.86 °F).

An individual's body temperature is set by the hypothalamus, an area in the base of the brain that acts like a thermostat, maintaining normal temperature through heating and cooling mechanisms. The main heating mechanism is the constriction of blood vessels in the periphery of the body, which decreases heat loss from the skin; another is shivering, which causes a release of heat from the muscles. Sweating acts as the body's chief cooling mechanism by causing the loss of heat through evaporation.

Pyrogens

Fever is the result of the hypothalamus raising the body's temperature set point in response to a stimulus (termed a pyrogen). In the cascade that leads to fever, pyrogens cause specific white blood cells called phagocytes to produce a number of proteins, including interleukins, interferons, and tumor necrosis factor alpha. These proteins then induce the production of prostaglandins, which subsequently leads to a resetting of the body's temperature set point.

In response to the elevation of the set point, the hypothalamus activates the heating mechanisms illustrated above, and the body's core temperature rises until the new set point is reached.

Most commonly, the inciting pyrogen comes from outside the body and, thus, is termed exogenous. Infectious agents (viruses or bacteria), medications, and drugs can all act as exogenous pyrogens. Occasionally, fever may be caused by a stimulus from within the body (an endogenous pyrogen); examples of conditions that produce these pyrogens are autoimmune and other inflammatory diseases, brain injury, and some forms of cancer.

Overall, infectious agents are by far the most common cause of fever. Examples of common viral and bacterial illnesses that cause fever are colds, the flu, and gastroenteritis. Additionally, some serious infections are known to cause fever, including meningitis, endocarditis, and sepsis; however, these conditions are relatively rare and usually have associated symptoms that indicate a significant underlying illness.

Typically, the pyrogen can be determined by a thorough history, physical exam, and laboratory testing as necessary.

Fever and the Immune System

Fever acts as a part of the body's defense system against infection. Most bacteria and viruses have evolved to function best with a surrounding temperature of around 37 °C (98.6 °F); therefore, the increase in ambient temperature that occurs with fever directly decreases the ability of those organisms to survive. Furthermore, fever activates the body's immune system, causing increased production of white blood cells, which in turn fight the infection that initiated the fever; in the presence of fever, these white blood cells also become more mobile, multiply faster, and are better able to destroy the invading microorganisms.

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