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A fever (or pyrexia) is a higher than normal body temperature. It is a symptom caused by a variety of illnesses.  A fever rarely comes without other symptoms. It is often accompanied by specific complaints, which may help to identify the illness causing the fever. This can help the doctor determine which treatment is necessary.


Every one of us has experienced the wave of chills and exhaustion that a fever causes. Fever usually occurs in response to an infection as with the flu virus or inflammation that occurs with tissue injury or disease. However, many other causes of fever are possible, including drugs, poisons, cancer, heat exposure, injuries or abnormalities to the brain, or disease of the endocrine (hormonal or glandular) system.

Normal body temperature can vary depending on the individual, the time of day, and even the weather. For most people, a temperature of 98.6 F (37 C) is baseline.


Temperature is usually controlled by the part of the brain called the hypothalamus. The hypothalamus is like a thermostat for the body. It maintains normal temperature through heating mechanisms, such as shivering and increased metabolism, and cooling mechanisms, such as sweating and dilating (opening) blood vessels close to the skin.


Fever occurs when the body's immune response is triggered by pyrogens (fever-producing substances). Pyrogens usually come from a source outside the body and, in turn, stimulate the production of additional pyrogens inside the body. Pyrogens tell the hypothalamus to increase the temperature set point. In response, our body begins to shiver; our blood vessels constrict (close); we get under the covers in an attempt to reach the new temperature that is higher than our baseline. However, other pyrogens can be produced by the body, usually in response to inflammation; these are referred to as cytokines (also termed endogenous pyrogens).


Pyrogens (fever-producing substances) that occur outside the body include the following:
Viruses
Bacteria
Fungi
Drugs
Toxins

Body temperature measurements are usually measured by temperature devices inserted on or into the rectum, mouth, axilla (under the armpit), skin, or ear. Some devices (laryngoscopes, bronchoscopes, rectal probes) may have temperature-sensing probes that can record temperature continually. The most common way to measure body temperature was (and still is in many countries) with a mercury thermometer; because of glass breakage and the possibility of subsequent mercury contamination, many developed countries use digital thermometers with disposable probe covers to measure temperature from all of the body sites listed above. 


Disposable temperature-sensitive strips that measure skin temperature are also used. Oral temperatures are most commonly measured in adults, but rectal temperatures are the most accurate because environmental factors that increase or decrease temperature measurements have the least effect on the rectal area. Rectal temperatures, when compared to oral temperatures taken at the same time, are about 1.8 F (0.6 C) higher. Consequently, an accurate measurement of body temperature (best is rectal core temperature) of 100.4 F (38 C) or above is considered to be a "fever."

Low-grade fevers range from about 100 F-101 F while high-grade fevers range from about 103 F-104 F. Dangerous temperatures are high-grade fevers that range from over 104 F-107 F or higher (extremely high fevers are also termed hyperpyrexia). The preceding fever values may vary somewhat according to different clinicians and the condition and age of the patient, but they offer a reader a way to judge the terms "low," "high," and "dangerous" when they are used in reference to fever in the medical literature.

Other types of fevers:

Prolonged fever is fever lasting longer than about 10-14 days.
Constant fever is also termed continuous fever; it is usually low-grade fever and does not change by much (by about 1 degree F over 24 hours).
Chronic: Fever lasts longer than three to four days; some researchers consider intermittent fevers that recur over months to years as "chronic" fevers.
Intermittent: Temperature either varies from normal to fever levels during a single day or fever may occur one day and recur in about one to three days.
Remittent: Fevers come and go at regular intervals
Hyperpyrexia: Fevers that are equal to or above 106.7 F that constitute a medical emergency.

In addition, there are well over 40 diseases that have "fever" as part of the disease name (for example, rheumatic fever, scarlet fever, cat scratch fever, Lassa fever, and many more). Each disease has fever as one of its symptoms; countless other conditions may have fever as a symptom.

Cytokines or endogenous (body-generated) pyrogens can cause many of the same features mentioned above. Cytokine release is triggered by inflammation and many immune-mediated diseases. People may have both infectious (also termed exogenous) pyrogens and cytokines generating fevers at the same time, depending on their disease processes. The major cytokines involved in fever generation are interleukins 1 and 6 along with tumor necrosis factor (TNF)-alpha.
Viral fever

Illnesses caused by viruses are among the most frequent causes of fever in adults. Symptoms can include a runny nose, sore throat, cough, hoarseness, and muscle aches. Viruses also may cause diarrhea, vomiting, or an upset stomach.

For the most part, these viral illnesses will improve simply with time. Antibiotics will not treat a virus. Symptoms can be treated using decongestants and anti-fever medications bought over the counter. If diarrhea or vomiting occurs, then the person needs to be encouraged to drink fluids. Gatorade or sports drinks will replace lost electrolytes. If fluids are not staying down, then medical care should be sought. Viral illnesses can last as long as one to two weeks.

The influenza virus is a major cause of death and serious illness in the elderly. Symptoms include headaches and muscle and joint aches, as well as the other common viral symptoms, including fever. Vaccines against seasonal influenza as well as H1N1 influenza are available. Also, antiviral medications can be administered to fight the influenza virus immediately after the symptoms start. This illness usually occurs during the winter.

Bacterial fever

Bacterial illnesses causing fever can affect almost any organ system in the body. They can be treated with antibiotics.

Central nervous system (brain and spinal cord) infections can cause fever, headache, neck stiffness, or confusion. A person may feel lethargic and irritable, and light may irritate the eyes. This could represent meningitis or a brain infection, so the person should go immediately to the doctor.
Lower respiratory system infections including pneumonia and bronchitis can cause fever. Symptoms include coughing, difficulty breathing, thick mucus production, and sometimes chest pain.


Upper respiratory system infections occur in the throat, ears, nose, and sinuses. A runny nose, headache, cough, or sore throat accompanied by a fever may indicate a bacterial infection, but a viral infection is the most common cause.
Infection of the genitourinary system may cause a person to have a burning sensation when urinating, blood in the urine, the urge to urinate frequently, and back pain along with a fever. This would indicate an infection in the bladder, kidney, or urinary tract. Antibiotics would treat such an infection.


If the reproductive system is affected, people often see a discharge from the penis or vagina and have pelvic pain along with the fever. Pelvic pain and fever in women may represent pelvic inflammatory disease (PID), which can cause significant damage to the reproductive organs. In this case, the person and any sexual partners should see a physician.


Gastrointestinal system (digestive system) infections are indicated by diarrhea, vomiting, stomach upset, and sometimes blood in the stool. Blood in the stool can indicate a bacterial infection or other type of serious illness. Abdominal pain may be caused by an infection of the appendix, gallbladder, or liver, and medical care should be accessed.
The circulatory system (including the heart and lungs) can be invaded by bacteria. There may not be any specific symptoms with the fever. A person may feel body aches, chills, weakness, or confusion. The condition known as sepsis is present when bacteria enter the bloodstream. An infection of a heart valve with resulting inflammation (endocarditis) can occur in people who had heart surgery in the past and in people who use IV drugs. This condition requires hospitalization and immediate treatment with IV antibiotics.


Skin, the largest organ in our body, can also be the source of a bacterial infection. Redness, swelling, warmth, pus, or pain occurs at the site of the infection. An infection may result from trauma to the skin or even a clogged pore that becomes an abscess. The infection can spread to the soft tissues beneath the skin (cellulitis). Sometimes the infection needs to be drained. Antibiotics are often needed. In addition, skin can react to some toxins by producing a skin rash; for example, the scarlatina rash that can occur after a Strep throat infection causes scarlet fever (skin rash is bright red and diffuse, with some skin that develops scaling and desquamation, or skin peeling off).

Fungal fever

Fungal infections can affect any organ system. Often a physician can identify these infections through a physical examination. Sometimes further testing is required and in rare instances, fungal fevers may require a biopsy to diagnose the infection. An antifungal medication will usually treat the infection.

Animal exposure fever

Certain people who work with animals can be exposed to rare bacteria that can cause fevers. In addition to the fever, the person may have chills, headache, and muscle and joint aches. These bacteria can exist in livestock, in unpasteurized dairy products, and in the urine of infected animals.

Travelers' fever

Anyone who travels, especially outside the United States, may develop fever after exposure to various new foods, toxins, insects, or vaccine-preventable diseases.

The only vaccines required by the U. S. and other countries for travelers at this time are for yellow fever and meningitis; these requirements depend on when and where people travel. Childhood vaccines such as those against measles, mumps, rubella, diphtheria, tetanus, and polio should be current prior to travel. Vaccines against hepatitis A, meningitis, and typhoid can be obtained before people travel to an area where exposure to those diseases is likely. The U.S. Centers for Disease Control and Prevention (CDC) can advise people on the current vaccines that are recommended or required for travel to various countries.

When traveling, consumption of contaminated water, uncooked vegetables, or unpasteurized dairy products can cause a low-grade fever and traveler's diarrhea. Bismuth subsalicylate (Pepto-Bismol), loperamide (Imodium), and certain antibiotics can help reduce symptoms but in some people may prolong the disease. The symptoms of abdominal cramping, nausea, vomiting, headache, and bloating should go away in three to six days. A fever higher than 101 F (38.3 C) or the presence of blood in the stool is an indication to go to a doctor immediately.

Insect bites are a common way that infections are spread in some countries. Malaria is a serious infection that can occur after a mosquito bite. The bitten person may have fevers that come and go every few days. A blood test must be done to make the diagnosis. In certain infected areas, a traveler can take medication to prevent malaria. Lyme disease is spread by the bite of a tick. This is common in areas of the U.S. where the deer tick is found. Any infection caused by an insect bite should be evaluated by a doctor.


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