(redirected from of unknown origin fever)
Also found in: Dictionary, Thesaurus, Encyclopedia.




A fever is any body temperature elevation over 100 °F (37.8 °C).


A healthy person's body temperature fluctuates between 97 °F (36.1 °C) and 100 °F (37.8 °C), with the average being 98.6 °F (37 °C). The body maintains stability within this range by balancing the heat produced by the metabolism with the heat lost to the environment. The "thermostat" that controls this process is located in the hypothalamus, a small structure located deep within the brain. The nervous system constantly relays information about the body's temperature to the thermostat, which in turn activates different physical responses designed to cool or warm the body, depending on the circumstances. These responses include: decreasing or increasing the flow of blood from the body's core, where it is warmed, to the surface, where it is cooled; slowing down or speeding up the rate at which the body turns food into energy (metabolic rate); inducing shivering, which generates heat through muscle contraction; and inducing sweating, which cools the body through evaporation.
A fever occurs when the thermostat resets at a higher temperature, primarily in response to an infection. To reach the higher temperature, the body moves blood to the warmer interior, increases the metabolic rate, and induces shivering. The "chills" that often accompany a fever are caused by the movement of blood to the body's core, leaving the surface and extremities cold. Once the higher temperature is achieved, the shivering and chills stop. When the infection has been overcome or drugs such as aspirin or acetaminophen (Tylenol) have been taken, the thermostat resets to normal and the body's cooling mechanisms switch on: the blood moves to the surface and sweating occurs.
Fever is an important component of the immune response, though its role is not completely understood. Physicians believe that an elevated body temperature has several effects. The immune system chemicals that react with the fever-inducing agent and trigger the resetting of the thermostat also increase the production of cells that fight off the invading bacteria or viruses. Higher temperatures also inhibit the growth of some bacteria, while at the same time speeding up the chemical reactions that help the body's cells repair themselves. In addition, the increased heart rate that may accompany the changes in blood circulation also speeds the arrival of white blood cells to the sites of infection.

Causes and symptoms

Fevers are primarily caused by viral or bacterial infections, such as pneumonia or influenza. However, other conditions can induce a fever, including allergic reactions; autoimmune diseases; trauma, such as breaking a bone; cancer; excessive exposure to the sun; intense exercise; hormonal imbalances; certain drugs; and damage to the hypothalamus. When an infection occurs, fever-inducing agents called pyrogens are released, either by the body's immune system or by the invading cells themselves, that trigger the resetting of the thermostat. In other circumstances, the immune system may overreact (allergic reactions) or become damaged (autoimmune diseases), causing the uncontrolled release of pyrogens. A stroke or tumor can damage the hypothalamus, causing the body's thermostat to malfunction. Excessive exposure to the sun or intensely exercising in hot weather can result in heat stroke, a condition in which the body's cooling mechanisms fail. Malignant hyperthermia is a rare, inherited condition in which a person develops a very high fever when given certain anesthetics or muscle relaxants in preparation for surgery.
How long a fever lasts and how high it may go depends on several factors, including its cause, the age of the patient, and his or her overall health. Most fevers caused by infections are acute, appearing suddenly and then dissipating as the immune system defeats the infectious agent. An infectious fever may also rise and fall throughout the day, reaching its peek in the late afternoon or early evening. A low-grade fever that lasts for several weeks is associated with autoimmune diseases such as lupus or with some cancers, particularly leukemia and lymphoma.


A fever is usually diagnosed using a thermometer. A variety of different thermometers are available, including traditional glass and mercury ones used for oral or rectal temperature readings and more sophisticated electronic ones that can be inserted in the ear to quickly register the body's temperature. For adults and older children, temperature readings are usually taken orally. Younger children who cannot or will not hold a thermometer in their mouths can have their temperature taken by placing an oral thermometer under their armpit. Infants generally have their temperature taken rectally using a rectal thermometer.
As important as registering a patient's temperature is determining the underlying cause of the fever. The presence or absence of accompanying symptoms, a patient's medical history, and information about what he or she may have ingested, any recent trips taken, or possible exposures to illness help the physician make a diagnosis. Blood tests can aid in identifying an infectious agent by detecting the presence of antibodies against it or providing samples for growth of the organism in a culture. Blood tests can also provide the doctor with white blood cell counts. Ultrasound tests, magnetic resonance imaging (MRI) tests, or computed tomography (CT) scans may be ordered if the doctor cannot readily determine the cause of a fever.


Physicians agree that the most effective treatment for a fever is to address its underlying cause, such as through the administration of antibiotics. Also, because a fever helps the immune system fight infection, it usually should be allowed to run its course. Drugs to lower fever (antipyretics) can be given if a patient (particularly a child) is uncomfortable. These include aspirin, acetaminophen (Tylenol), and ibuprofin (Advil). Aspirin, however, should not be given to a child or adolescent with a fever since this drug has been linked to an increased risk of Reye's syndrome. Bathing a patient in cool water can also help alleviate a high fever.
A fever requires emergency treatment under the following circumstances:
  • newborn (three months or younger) with a fever over 100.5 °F (38 °C)
  • infant or child with a fever over 103 °F (39.4 °C)
  • fever accompanied by severe headache, neck stiffness, mental confusion, or severe swelling of the throat
A very high fever in a small child can trigger seizures (febrile seizures) and therefore should be treated immediately. A fever accompanied by the above symptoms can indicate the presence of a serious infection, such as meningitis, and should be brought to the immediate attention of a physician.


Most fevers caused by infection end as soon as the immune system rids the body of the pathogen and do not produce any lasting effects. The prognosis for fevers associated with more chronic conditions, such as autoimmune disease, depends upon the overall outcome of the disorder.



Gelfand, Jeffrey. "Fever, Including Fever of Unknown Origin." In Harrison's Principles of Internal Medicine, edited by Anthony S. Fauci, et al. New York: McGraw-Hill, 1997.

Key terms

Antipyretic — A drug that lowers fever, like aspirin or acetaminophen.
Autoimmune disease — Condition in which a person's immune system attacks the body's own cells, causing tissue destruction.
Febrile seizure — Convulsions brought on by fever.
Malignant hyperthermia — A rare, inherited condition in which a person develops a very high fever when given certain anesthetics or muscle relaxants in preparation for surgery.
Meningitis — A potentially fatal inflammation of the thin membrane covering the brain and spinal cord.
Metabolism — The chemical process by which the body turns food into energy, which can be given off as heat.
Pyrogen — A chemical circulating in the blood that causes a rise in body temperature.
Reye's syndrome — A disorder principally affecting the liver and brain, marked by the rapid development of life-threatening neurological symptoms.
Gale Encyclopedia of Medicine. Copyright 2008 The Gale Group, Inc. All rights reserved.


1. an abnormally high body temperature; called also pyrexia. adj., adj fe´brile, fe´verish.
2. any disease characterized by marked increase of body temperature. For specific diseases, see the eponymic or descriptive name, such as rocky mountain spotted fever or typhoid fever. Other conditions involving elevated body temperature include heat exhaustion and heat stroke.

Normal body temperature when the body is at rest is 37°C (98.6°F). This is an average or mean body temperature that varies from person to person and from hour to hour in an individual. The route by which a body temperature is measured affects the reading. The normal oral temperature ranges from 36° to 37.5°C (96.8° to 99.5°F). If the temperature is measured rectally, the norm would be 0.5°C (1°F) higher. An axillary temperature would be 0.5°C (1°F) lower. Because of these differences, the number should always be followed by the route by which the temperature was taken when the reading is recorded.

Factors that can cause a temporary elevation in body temperature include age, physical activity, emotional stress, and ovulation. If a person has a consistently elevated temperature, fever is said to exist. A low-grade fever is marked by temperatures between 37.5° and 38.2°C (99.5° and 101°F) when taken orally. A high-grade fever is present when the oral temperature is above 38.2°C (101°F).

Types of fever include continued or continuous fever, one lasting more than 24 hours without significant variation or any return to normal body temperature; intermittent fever, in which at least once during a 24-hour period the fever spikes are separated by a return to normal body temperature; remittent fever, in which elevated body temperature shows fluctuations each day but never returns to normal; and recurrent (or relapsing) fever, in which periods of fever and normal body temperature alternate and last about 5 to 7 days each.

The regulation of body temperature is under the control of the hypothalamus. Thermolysis, or dissipation of body heat, is regulated by the anterior hypothalamus in conjunction with the parasympathetic nervous system. The overall effect of heat loss is accomplished by vasodilation of the peripheral blood vessels, increased sweating, and decreased metabolic and muscular activities. The production and conservation of body heat, or thermogenesis, is regulated by the posterior hypothalamus in conjunction with the sympathetic nervous system. The mechanisms by which body heat is produced and conserved are in opposition to those that increase heat loss; that is, by constriction of cutaneous blood vessels, decreased sweat gland activity, and increased metabolic and muscular activities.

Fever develops when there is some disturbance in the homeostatic mechanisms by which the hypothalamus maintains a balance between heat production and peripheral heat loss. Although dehydration, cerebral hemorrhage, heat stroke, thyroxine, and certain other drugs can cause an elevated body temperature or hyperthermia, fever, in the precise sense of the term, occurs as a result of inflammation or infection, or both. During the infectious and inflammatory processes certain substances called pyrogens are produced within the body. These endogenous pyrogens are the result of inflammatory reactions, such as those that occur in tissue damage, cell necrosis, rejection of transplanted tissues, malignancy, and antigen-antibody reactions. Exogenous pyrogens are introduced into the body when it is invaded by bacteria, viruses, fungi, and other kinds of infectious organisms.

Endogenous pyrogens act directly on the hypothalamus, affecting its thermostatic functions by “resetting” it to a higher temperature. When this happens, all of the physiologic activities concerned with heat production and conservation operate to maintain body temperature at a higher setpoint. The symptoms of chill and shivering are the result of increased muscular activity, which is an attempt by the body to raise its temperature to the higher setting. This increased muscular activity is accompanied by an elevation of the metabolic rate, which in turn increases the demand for nutrients and oxygen. Outward signs of these internal activities include a higher pulse rate, increased respirations, and thirst caused by the loss of extracellular water via the lungs. The pulse rate increases at the rate of about eight to ten beats per minute for each degree of temperature rise.

Once the body temperature reaches the setpoint of the hypothalamic thermostat, the mechanisms of heat production and heat loss keep it at a fairly constant level and the fever persists. This is sometimes called the second stage of fever. If it continues, fluid and electrolyte losses become more severe and there is evidence of cellular dehydration. During this stage delirium in older persons and convulsions in infants and children can occur. Febrile convulsions in children are believed to be closely related to cerebral damage that becomes evident as afebrile convulsions later in life.

Prolonged fever eventually brings about tissue destruction owing to the catabolism of body proteins. Because of this the patient experiences muscle aches and weakness, malaise, and the excretion of albumin in the urine. Anorexia also is present. If the body does not receive a sufficient supply of energy from dietary intake to meet its metabolic needs, it catabolizes its own fat and protein. The patient then rapidly loses weight and can develop ketosis and metabolic acidosis.

The period during which a fever abates is called the period of defervescence. It may occur rapidly and dramatically, as the temperature falls from peak to normal in a matter of hours. This is called the crisis, that is, the critical point at which the fever is broken. A more gradual resetting of the thermostat and slow decline of the fever is called resolution of the fever by lysis.
Treatment. It is not always necessary to reduce fever and in many cases it may be best not to treat it, at least until its cause is determined. The fever pattern can provide diagnostic information and is not necessarily harmful unless it is extremely high or the patient has cardiac or respiratory disease and cannot tolerate the additional tachycardia and dyspnea that may accompany fever. An elevated body temperature can inhibit bacterial replication and the action of viruses, spirochetes, and other pathogenic microorganisms.

If it is decided that treatment is necessary, there are two major goals: to identify the cause and to provide symptomatic relief. Antipyretic drugs such as aspirin and acetaminophen (Tylenol) are generally safe and effective. However, acetaminophen is preferred in children and when the patient has gastrointestinal sensitivity, allergy to aspirin, or a clotting disorder or is suspected of having Reye's syndrome.

Fluids and electrolytes are replaced orally or intravenously as indicated by laboratory tests and signs of dehydration. Frequent, small feedings of high-calorie, high-protein foods are recommended to combat fatigue and debility caused by the increased metabolic rate. The selection of oral liquids and foods should be based on the patient's preferences. Vitamin supplements may be prescribed in prolonged, low-grade fevers.
Patient Care. The patient with acute hyperpyrexia or hyperthermia will require extreme measures to lower the body temperature as quickly and safely as possible in order to prevent brain damage. Victims of heat stroke should be cooled rapidly. In order to keep the temperature at a tolerable level until the thermostat is reset, a cooling blanket or hypothermia mattress may be used. Care must be taken to maintain the integrity of the skin and avoid sudden and extreme hypothermia when such a device is used. Other measures include sponging parts of the body with cool water to increase heat loss through evaporation of moisture. The part being sponged should be left exposed to the air until it is almost dry, and then should be lightly covered while another part is being sponged. A cold compress on the forehead helps to reduce the fever and relieve headache and delirium. An alternative to sponging and a cool bath is the application of ice packs to specific parts of the body, such as the abdomen, groin, axillae, and spine. Fanning can also be effective, especially if the patient's torso is covered with a sheet saturated with water.

Chills are uncomfortable and sometimes frightening to the patient. When the patient complains of feeling chilled or cold, some form of external warmth should be provided. An extra blanket is helpful as is a hot water bottle filled with warm, not hot, water. As the body temperature declines the difference between body temperature and environmental temperature will decrease and the patient will begin to feel warmer. During the second stage of fever the patient may complain of feeling hot; the skin feels warm to the touch and the face is flushed. These symptoms are the result of vasodilation of surface blood vessels, an attempt by the body to prevent further escalation of the body temperature.
fever of unknown origin (FUO) a febrile illness of at least three weeks' duration with a temperature of at least 38.3°C on at least three occasions and failure to establish a diagnosis in spite of intensive inpatient or outpatient evaluation (three outpatient visits or three days' hospitalization). The duration of febrile illness needed to establish a diagnosis of FUO varies among authorities and is sometimes given as shorter than three weeks.

Classic fever of unknown origin, as defined by the preceding criteria, is distinguished from neutropenic and nosocomial FUO, as well as that associated with human immunodeficiency virus infection. In the neutropenic form, fever is accompanied by a neutrophil level that is lower than 500/mm3 or is expected to fall below that level within one or two days. The nosocomial form is a fever that occurs on several occasions in a hospitalized patient in whom neither fever nor infection was present on admission. In HIV-associated FUO, fever occurs in a person with human immunodeficiency virus infection on several occasions over a period of four weeks of outpatient care or three days of hospitalization. In all three of these forms of FUO, the cause of the fever cannot be determined after three days of investigation, including two days of incubation of cultures.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.


A complex physiologic response to disease mediated by pyrogenic cytokines and characterized by a rise in core temperature, generation of acute phase reactants, and activation of immune systems.
Synonym(s): febris, pyrexia
[A.S. fefer]


Farlex Partner Medical Dictionary © Farlex 2012


a. Abnormally high body temperature.
b. Any of various diseases characterized by abnormally high body temperature.
v. fe·vered, fe·vering, fe·vers
To effect fever in.
To be or become feverish.
The American Heritage® Medical Dictionary Copyright © 2007, 2004 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.


Internal medicine A body temperature of ≥ 37.2ºC/99.0ºF in early morning or ≥ 37.8ºC/100.0ºF in the evening that is a complex and coordinated adaptive response, which is part of the reaction to an immune challenge; this response is stereotyped, and largely independent of a causitive agent; as with other integrated responses–eg, regulation of energy metabolism, BP and volume, and reproduction, fever depends on humoral cues and is orchestrated by the hypothalamus, which coordinates autonomic, behavioral, endocrine, and metabolic responses– when corporal temperature is raised by endogenous pyrogen, T cell production ↑ 20-fold; endogenous pyrogen also shifts iron–needed by bacteria away from plasma; hyperthermia–up to 40ºC may trigger CA regression. See 'Animal House fever' Argentine hemorrhagic fever, Blackwater fever, Blue fever, Breakbone fever, Bolivian hemorrhagic fever, Bullis fever, Cabin fever, Cat-bite fever, Coley's toxin, Colorado tick fever, Congo-Crimean hemorrhagic fever, Cotton fever, Dehydration fever, Dengue hemorrhagic fever, Epidemic louse-borne typhus fever, Familial hibernian fever, Familial Mediterranean fever, Filarial fever, Fort Bragg fever, Haverhill fever, Hay fever, Hectic-septic fever, Hypothalamic fever, Izumi fever, Korean hemorrhagic fever, Lassa fever, Malta fever, Mediterranean fever, Metal fume fever, Omsk hemorrhagic fever, O'nyong-nyong fever, Polymer fume fever, Pontiac fever, Potamic fever, Q fever, Quartan fever, Quintan fever, Rat-bite fever, Relapsing fever, Rheumatic fever, Rift Valley fever, Rocky Mountain spotted fever, Sandfly fever, Scarlet fever, Seven-day fever, Simian hemorrhagic fever, Spring fever, Swamp fever, Tertian fever, Thirteen day fever, Three day fever, Trench fever, Valley fever, West Nile fever, Yellow fever.
McGraw-Hill Concise Dictionary of Modern Medicine. © 2002 by The McGraw-Hill Companies, Inc.


A complex physiologic response to disease mediated by pyrogenic cytokines and characterized by a significant rise in core temperature, generation of acute phase reactants, and activation of immunologic systems.
Synonym(s): pyrexia.
[A.S. fefer]
Medical Dictionary for the Health Professions and Nursing © Farlex 2012


Elevation of body temperature above about 37 C, taken in the mouth. Fever is due to a resetting of the body's thermostat at a higher level so that heat production, mainly by shivering, is induced. The resetting is caused by the CYTOKINE interleukin-1 produced by white cells under the influence of bacteria, cancer, CORONARY THROMBOSIS, STROKE, crushing injury and other conditions. Fever inhibits the growth of bacteria and causes an increase in antibody production. The recognition of these advantages has led to a general abandonment of the former practice of routinely trying to reduce moderate fever.
Collins Dictionary of Medicine © Robert M. Youngson 2004, 2005


A complex physiologic response to disease mediated by pyrogenic cytokines and characterized by a rise in core temperature, generation of acute phase reactants, and activation of immune systems.
Synonym(s): pyrexia.
[A.S. fefer]
Medical Dictionary for the Dental Professions © Farlex 2012

Patient discussion about fever

Q. High Fever - is it dangerous? high fever over a long period can be dangerous. on the other hand it shows also that your body is working like Superman...

A. the heart surgeon knows, that if he touchs during a heart-operation this sparkle of 96°C, the patient will die. exitus for sure! the problem for the heart surgeon is, that he don't know why it is so. this is the big problem you can have with a doctor who don't know about such things. if you like to know more about this sparkle and why the patient dies, write me a mail and i will share with you a link to read about it. ask me if you don't understand me for what i try to teach you. FEVER is a healing-system nature gave us. Let it work naturally. don't make a tonsillectomy just because some do or because it is a fashion. think first about it. your tonsils have functions and we need them. they are not just decoration. my parents forced me to make a tonsillectomy because they were told to do so because i had many otitis. i would never do it to my kids. i have since 30 years from time to time pain there and i loose some blood. it hurts terribly when it is acute.

Q. What Causes Cough With Fever? Often during winter I become sick with fever and on going cough. What causes it?

A. Cough with fever can be a first sign to many respiratory tract infections with viruses or bacterias, starting from simple ones as the common cold (or "flu") up to more serious conditions such as pneumonia, that requires medical care and medications.

Q. What Treats Cough With Fever? I have the flu and so my entire body hurts, and I can't stop coughing. What type of medications can I buy over-the-counter to help me feel better?

A. Simple analgesics are good for lowering fever and easing pain. As for the cough, there is a question whether or not cough suppressants are actually recommended. http://www.5min.com/Video/Do-Cough-Syrups-Work-5602143

More discussions about fever
This content is provided by iMedix and is subject to iMedix Terms. The Questions and Answers are not endorsed or recommended and are made available by patients, not doctors.