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elevation of body temperature above the normal level, which in humans is about 98°F; (37°C;) when measured orally. Fever is considered to be a symptom of a disorder rather than a disease in itself. Under normal conditions the heat that is generated by the burning of food by the body is dissipated through such processes as perspiration and breathing. It is believed that infectious diseases, injury to the body tissues, and other conditions that cause inflammation lead to the release of prostaglandins, a type of hormone, which bind to sites in the hypothalamus, the center of temperature control in the body. The rise in temperature that is triggered as a result acts as part of the body's defenses against infection; white blood cells become more active, and most bacteria do not thrive as well. The effects of fever on the body are weakness, exhaustion, and sometimes a depletion of body fluids through excessive perspiration. Extremely high fevers may cause convulsive reactions and eventual death. In addition to infectious diseases (such as pneumonia and tonsillitis), disorders of the brain, certain types of cancer, and severe heatstroke may cause fever. There are also cases of fever where the cause cannot be detected. Treatment includes increasing the intake of fluids and administering aspirin and other fever-reducing medications. Aspirin may be dangerous in fevers of children because of Reye's syndrome. However, primary treatment is directed at the underlying cause unless the fever is very high (above 104°F;/40°C;). Persons with such dangerously high fevers are sometimes sponged with cool water or immersed in cool baths.



a protective-adaptive body reaction in higher animals and man, expressed in the abnormal elevation of body temperature, which developed during the course of evolution.

In the past, all illnesses accompanied by high temperatures were called fevers. Although modern science shows that fevers are not independent diseases, the names of a number of diseases have retained the term (as, for example, in Q fever, phlebotomus fever, and metal fume fever). Fever is the reaction of the thermoregulatory apparatus to those macromolecular substances, called pyrogens, that affect the chemoreceptors. Pyrogens are classified as either exogenous, that is, entering the body from without (for example, microbes and the products of their life activities), or endogenous, that is, formed in the body itself (for example, products secreted by leukocytes in the process of phagocytosis or substances that appear as a result of damage to the cell-tissue structure).

The temporary reorganization of the regulatory mechanism of heat exchange in order to maintain the higher body temperature (as, for example, in the retention of the capacity to regulate temperature at a new level during muscular work or temperature fluctuations in the external environment) distinguishes fever from thermoregulatory disturbance, such as hyperthermia.

A special form of fever is the elevation of temperature when there has been extraordinary irritation of the sensory nerve endings in a number of organs (bile ducts, gall bladder, urethral canal) or damage to certain sections of the central nervous system (mechanical damage to the hypothalamic region). Elevated temperature is sometimes also observed to accompany mental disturbances. Although it is impossible to identify temperature changes with fever fully in all cases, the character of such changes is in many ways similar to that of fever.

There are three stages in the course of a fever: the rise, the leveling off, or acme, and the fall. In the first stage, heat production

Figure 1. Body-temperature, thermogenesis, and heat-loss curves with fever (I, II, III) and during muscular work (IV): (1) body temperature, (2) thermogenesis, (3) heat loss, (4) changes in thermal balance at various stages of fever. Crosshatched triangles indicate periods of accumulation and reduction of excess heat.

(thermogenesis) exceeds heat loss, mainly because of a decrease in the latter; in the second stage, equilibrium is established between heat formation and heat loss, the thermal balance shifting to a new, higher level; in the third stage, heat loss exceeds heat formation, mostly because of a sharp intensification in the former (see Figure 1). The fall in temperature in the third stage can occur gradually (lysis) or abruptly and rapidly (crisis). A critical fall in temperature, as a result of a concomitant abrupt dilation of the peripheral vessels, can lead to acute vascular insufficiency, or collapse.

When there is fever, the activities of all organs and systems are reorganized and the metabolism is altered. This is due not only to the elevated body temperature but also to the pathogenesis of the primary disease. In addition, the decomposition of proteins and the oxidation of carbohydrates and fats are intensified, and water and certain electrolytes tend to be retained in the body. Respiration and the rhythm of heart contractions are accelerated. In the first stage, the peripheral vessels are constricted; in the second and third stages, they are dilated. Glandular secretion in the digestive tract is suppressed. Disturbances of the activity of the central nervous system may be observed with particularly high fevers.

Fever is classified, according to the degree of temperature rise, as subfebrile (increases to 37.1°–37.9°C), moderate (38°–39.5°C), high (39.6°–40.9°C), and hyperpyretic (41°C and higher; in man, to 42°C). During the second stage, the fever may be characterized as continued, where daily fluctuations do not exceed 1°C (for example, with lobar pneumonia; see Figure 2); remittent, with daily fluctuations as great as 2°C (for example, certain forms of tuberculosis); intermittent, with falls during the daily minimum to normal or below normal; or hectic, with daily fluctuations as great as 3°–4°C (for example, with sepsis or severe, progressive tuberculosis). A disturbance of the diurnal temperature rhythm (a rise in temperature in the morning hours and a decrease in the evening) is characteristic of a perverted, atypical fever.

Figure 2. Temperature curve (continued type) in lobar pneumonia

Fever stimulates the immune processes and suppresses the action of the infectious agent. For this reason, artificially induced fever is sometimes used in medical practice (for example, in the fourth stage of syphilis, with schizophrenia, and with epidemic encephalitis).


Veselkin, P. N. Likhoradka. Moscow, 1963.
Mnogotomnoe rukovodstvo po patologicheskoi fiziologii, vol. 2. Moscow, 1966.



An elevation in the central body temperature of warm-blooded animals caused by abnormal functioning of the thermoregulatory mechanisms.


1. an abnormally high body temperature, accompanied by a fast pulse rate, dry skin, etc.
2. any of various diseases, such as yellow fever or scarlet fever, characterized by a high temperature