How does fever occur?

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Fever can be produced by a group of either endogenous or exogenous substances called pyrogens.

Exogenous (external) pyrogens derive from microbes and their toxins which stimulate cells found in the immune system (macrophages and others) to synthesise endogenous (internal) pyrogens, or cytokines. These cytokines subsequently induce the production of prostaglandin E2 (PGE2). PGE2 causes peripheral vasoconstriction, an increase in metabolism and muscle contractions which all lead to a rise in body temperature.

When the fever process starts, the hypothalamic set point increases, yet body temperature remains normal, which means the patient experiences chills and shivers. The shivering, or muscle contractions, elevates the body temperature and stops the chills. When the hypothalamic set point drops to a lower value, the patient starts feeling hot and begins to sweat. Sweating helps reduce the body’s temperature back to a normal value.

Causes of Fever

The main cause of fever is an infection. Fever is a general response to an infectious process. It is a coping mechanism that attempts to fight off the infection by decreasing the rate of microbe reproduction and increasing the inflammatory response.

When an infection enters the body, its defensive cells (white blood cells) fight against the invading agent by producing a series of molecules (cytokines or endogenous pyrogens) that are transported in the blood until they reach the thermoregulatory centre in the hypothalamus, which changes the set-point temperature of its thermostat. Therefore, at first patients usually notice intense cold, which is sometimes accompanied by chills, then later they feel hot and sweat heavily, all of which are symptoms caused by the change in body temperature.

Temperatures above 41 °C are rarely due to an infectious process. They are normally the result of a central cause, in other words, a change in central nervous system function. Temperatures of this magnitude are harmful to the body. Patients suffer confusion, hallucinations and drowsiness. At even higher temperatures, the body’s proteins undergo structural alterations and it may cause irreversible damage. At 42 °C the individual will fall into a coma and a body temperature of 43 °C leads to death.