- Also called:
- pyrexia
- Key People:
- Theodor Billroth
- Related Topics:
- Ebola
- yellow fever
- chikungunya fever
- dengue
- scarlet fever
- On the Web:
- National Center for Biotechnology Information - Physiology, Fever (Nov. 06, 2026)
fever
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What is a fever?
A fever is defined as an abnormally high body temperature, typically above 99°F (37.22°C). It is a common symptom of various illnesses, most often infections. However, it can also arise from cancer, coronary artery occlusion, blood disorders, intense exercise, ovulation, heat exhaustion, or heat stroke.
During a fever, the body's blood and urine volumes decrease due to increased perspiration. Body protein breaks down rapidly, leading to increased excretion of nitrogenous products in the urine.
The hypothalamus in the brain regulates body temperature. Pyrogens, produced by the immune system in response to tissue injury, interfere with the hypothalamus, raising the body's temperature to combat microbial invasion.
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How does the body develop a fever?
A fever is the body's defensive reaction to tissue injury caused by invading bacteria or viruses. The immune system responds by producing pyrogens, chemicals carried by the blood to the brain, where they interfere with the hypothalamus' function. The hypothalamus regulates body temperature.
Pyrogens inhibit heat-sensing neurons and excite cold-sensing ones, deceiving the hypothalamus into thinking the body is cooler than it is. In response, the hypothalamus raises the body’s temperature above the normal range, causing a fever. This helps defend against microbial invasion by stimulating white blood cell activity and antibody production. Elevated heat levels may also directly kill or inhibit the growth of some bacteria and viruses.
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What are common causes of fever in people?
Fever, an abnormally high body temperature, is a common symptom triggered by various underlying causes. Infections, whether from viruses, bacteria, or other microorganisms, are frequent culprits. When these pathogens invade the body, the immune system releases pyrogens, which interfere with the hypothalamus' temperature regulation, leading to a rise in body temperature.
However, fever isn't always due to infection; it can also arise from conditions like cancer, coronary artery occlusion, blood disorders, strenuous exercise, ovulation, heat exhaustion, or heat stroke. Inflammation, marked by redness, heat, swelling, and pain, can also contribute to fever. Determining the underlying cause is crucial for effective treatment.
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How does a fever help the body fight infection?
A fever is a defensive reaction by the body against infection. When bacteria or viruses invade the body, the immune system produces pyrogens that travel to the brain and disturb the hypothalamus, which regulates body temperature. This causes the hypothalamus to raise the body's temperature, resulting in a fever.
The elevated temperatures help defend against microbial invasion by stimulating the activity and multiplication of white blood cells and increasing antibody production. Higher temperatures may also directly kill or inhibit the growth of some bacteria and viruses. Additionally, interleukins released by macrophages raise body temperature and stimulate liver cells to secrete acute-phase proteins, which activate complement proteins that destroy pathogens.
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What symptoms usually occur along with a fever?
A fever, defined as an abnormally high body temperature, is associated with several symptoms. When a fever occurs, the person affected may experience reduced blood and urine volumes as a result of water loss through increased perspiration.
Other symptoms that may occur along with a fever:
* Weakness or exhaustion
* Chills
* Muscle aches
* Headache
* Sore throatIn treating a fever, it is important to determine its underlying cause.
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When can a fever become dangerous and require medical attention?
A fever becomes dangerous when body temperature rises to 108 °F (42.22 °C) or more, potentially leading to convulsions and even death. Mild to moderate fevers, up to 105 °F (40.55 °C), typically cause weakness or exhaustion but are not an immediate threat.
It is important to determine the underlying cause of the fever. In cases of infection, lower grade fevers can be left untreated to allow the body to fight off microorganisms. Higher fevers may require treatment with medications like acetaminophen or ibuprofen.
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fever, abnormally high body temperature. Fever is a characteristic of many different diseases. For example, although most often associated with infection, fever is also observed in other pathologic states, such as cancer, coronary artery occlusion, and certain disorders of the blood. It also may result from physiological stresses, such as strenuous exercise or ovulation, or from environmentally induced heat exhaustion or heat stroke.
Under normal conditions, the temperature of deeper portions of the head and trunk does not vary by more than 1–2 °F in a day, and it does not exceed 99 °F (37.22 °C) in the mouth or 99.6 °F (37.55 °C) in the rectum. Fever can be defined as any elevation of body temperature above the normal level. Persons with fever may experience daily fluctuations of 5–9 °F above normal; peak levels tend to occur in the late afternoon. Mild or moderate states of fever (up to 105 °F [40.55 °C]) cause weakness or exhaustion but are not in themselves a serious threat to health. More serious fevers, in which body temperature rises to 108 °F (42.22 °C) or more, can result in convulsions and death.
During fever the blood and urine volumes become reduced as a result of loss of water through increased perspiration. Body protein is rapidly broken down, leading to increased excretion of nitrogenous products in the urine. When the body temperature is rising rapidly, the affected person may feel chilly or even have a shaking chill; conversely, when the temperature is declining rapidly, the person may feel warm and have a flushed moist skin.
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In treating fever, it is important to determine the underlying cause of the condition. In general, in the case of infection, low-grade fevers may be best left untreated in order to allow the body to fight off infectious microorganisms on its own. However, higher fevers may be treated with acetaminophen or ibuprofen, which exerts its effect on the temperature-regulating areas of the brain.
The mechanism of fever appears to be a defensive reaction by the body against infectious disease. When bacteria or viruses invade the body and cause tissue injury, one of the immune system’s responses is to produce pyrogens. These chemicals are carried by the blood to the brain, where they disturb the functioning of the hypothalamus, the part of the brain that regulates body temperature. The pyrogens inhibit heat-sensing neurons and excite cold-sensing ones, and the altering of these temperature sensors deceives the hypothalamus into thinking the body is cooler than it actually is. In response, the hypothalamus raises the body’s temperature above the normal range, thereby causing a fever. The above-normal temperatures are thought to help defend against microbial invasion because they stimulate the motion, activity, and multiplication of white blood cells and increase the production of antibodies. At the same time, elevated heat levels may directly kill or inhibit the growth of some bacteria and viruses that can tolerate only a narrow temperature range.