Fever is an abnormally elevated body temperature that usually occurs during an infection, inflammation, or some other kind of illness. Fever is not a disease itself, but it is one of the most common signs of illness, especially among children.

How Is Body Temperature Controlled?

The body adjusts its temperature in much the same way that the thermostat in a house works. With a thermostat, people set the temperature they want, and the heating or cooling system clicks on until the inside of the house reaches the right temperature. After that, the heater or air conditioner clicks on and off automatically to keep the temperature in the house hovering around the desired temperature.

The body's thermostat is located in the hypothalamus (hy-po-THALa-mus), a small part of the brain that also helps to regulate hunger, thirst, awareness of pleasure, and awareness of pain. The thermostat region of the hypothalamus, called the thermoregulatory (ther-mo-REG-u-la-toree) center, normally keeps the body's temperature hovering at around 98.6° F (36.8° C).

Ups and Downs

A person's temperature normally varies in the course of a day by about 1° F (0.6° C). It is lowest in the early morning and highest in the late afternoon. This daily variation is called the circadian (sir-KADE-ee-an) rhythm. When a person has a fever, it usually follows the same daily pattern.

Other factors also can affect what is a normal body temperature. In women of childbearing age, for instance, the early morning temperature usually goes up each month just before ovulation (ov-u-LA-shun), the release of an egg from the ovary. It stays elevated briefly and then returns to its usual level.

Like a house, the body has sensors that tell the thermostat if the temperature inside is rising or falling. In the body, these sensors are cells located in the skin and in the brain itself. If the sensors report that the body's temperature is rising, the body's cooling system clicks on, telling the cells to burn less fuel and produce less heat. The blood vessels expand to let heat escape from the skin (the evaporating sweat draws heat away from blood vessels), sweat pours out to cool the body as it evaporates, and the brain may get a bright idea: “Let's go into the shade and have a cold drink.”


With fever, the thermostat in the brain is reset to a higher temperature. Instead of keeping the body's temperature hovering at around 98.6° F, the body's heating and cooling systems may keep the temperature at 100° F to 102° F or even higher.

Normal temperature varies a bit from person to person and from morning to evening, making it hard to state precisely where normal ends and fever begins. In fact, temperature measured at different places in the body can also be different. Temperature measured at the mouth can be one whole degree Fahrenheit lower than the actual core body temperature. For this reason, core body temperature is best measured at the rectum or at the ear. Most doctors say that a core body temperature at or above 100.4° F (38° C) should be considered a fever. Core body temperature above 104° F (40° C) is generally considered a high fever.


Many infectious diseases are named for the major symptom of fever. Most of those listed lead to fevers of about 102° F to 104° F

(39° C to 40° C). Dengue fever, Lassa fever, and yellow fever are caused by viruses. The others are caused by bacteria.

Fever of Unknown Origin

Sometimes a person has a fever that lasts for two to three weeks, and the doctor cannot find a cause, despite performing the usual array of medical tests. This condition is referred to as fever of unknown origin.

In about 90 percent of cases, medical professionals eventually find a cause. The most common causes are infectious diseases. Fever of unknown origin is particularly common in people infected with human immunodeficiency virus (HIV), the virus that causes acquired immunodeficiency syndrome (AIDS).


Sometimes a person's temperature can rise for a different reason. Hyperthermia (hy-per-THER-me-a), which is quite different from fever, occurs when the heat outside is too much for the body's cooling system to handle, making body temperature (and the set point) rise. The most severe cases of hyperthermia tend to occur in people who cannot sweat as much as others, such as elderly people or those taking certain medications.

How Does Illness Cause Fever?

Bacteria and viruses themselves, as well as toxins (poisonous waste products) produced by some bacteria, cause fever. In some cases, they work directly on the brain to raise the thermostat. More commonly, they cause the body's immune system * to produce proteins called cytokines (SY-to-kines). The cytokines help fight the infection, but they also reset the brain's thermostat, causing fever.

Any substance that causes fever is called a pyrogen (PY-ro-jen), from the Greek word for “fire-causer.” If the substance comes from outside the body, such as a toxin from bacteria, it is called an exogenous (ekSOJ-e-nus) pyrogen. The prefix exo means “outside” in Greek. If the substance comes from inside the body, such as a cytokine, it is called an endogenous (en-DOJ-e-nus) pyrogen. The prefix endo means “inside” in Greek.

Sometimes the immune system produces pyrogens even without an infection. For instance, this may happen if a person has the following:

People sometimes say that fever is a sign that the immune system is active, working to protect the body from illness. That may be true in some cases, but it is not always so. People often get fevers if their immune system is weak or damaged. Scientists are not sure exactly what, if anything, fever indicates about the state of the immune system.

Who Gets Fever?

Fever is caused by so many common illnesses, including colds and flu, that people usually experience fevers during the course of their life. Young children are particularly likely to get bacterial and viral infections, such as strep throat and ear infections, that cause fever. Sometimes minor viral infections cause high fevers in children, whereas illnesses that are more serious cause milder fevers. People of all ages experience fevers.

Helpful fever

Some evidence suggests that an elevated body temperature can make the immune system more effective and weaken certain bacteria. However, most of this evidence comes from animals or experiments on human cells in test tubes. Scientists really do not know whether fever actually helps people fight infections. It may be that fever helps in certain cases but not in others.

Fever often can be helpful in another way. It can be an important sign that a person is sick. The movements of temperature up and down can indicate whether a person is getting better or worse.

Harmful fever

Fever often makes an illness more unpleasant. In addition, a feverish body needs more oxygen, which means that the heart and lungs have to work harder as the fever rises, which can be a problem for people who already have heart or lung problems.

Fever can make mental problems worse for elderly people who have dementia (de-MEN-sha), which is a form of mental confusion and loss of memory that can develop gradually as people age. High fever also can cause temporary mental confusion, called delirium (de-LEER-e-um), even in healthy people.

Febrile convulsion

Children under five years of age may experience a kind of seizure * called a febrile convulsion (FEB-ryl kon-VUL-shun) if their temperature rises quickly. Their muscles may twitch, and they may lose consciousness for several minutes. Usually, a febrile convulsion needs no treatment and may not recur. However, febrile convulsions can be frightening. They also can lead to injury; for example, if a child falls.

Extremely high temperatures of around 107° F or higher can cause permanent brain damage at any age if they last for a long time. Temperatures that high usually result from hyperthermia, not from an illness-related fever.

How Is Fever Diagnosed?

People with a fever often feel hot, tired, achy, and generally sick. They sometimes have shaking chills as their temperature rises. Shaking helps raise the temperature to the feverish level that has been set by the body's thermostat. They may sweat heavily when the fever “breaks” (starts to go away) or if it falls temporarily as part of an up-and-down pattern. Sweating helps lower the temperature to the new, lower point set by the thermostat.

Although the classic way of checking for fever at home is to touch the person's forehead or feel under the arm to see how warm it feels, doing so often does not work. The only way to tell for sure if a person has a fever is by taking the temperature with a thermometer. Four kinds of thermometers are available: digital, mercury, thermal, or tympanic.

Fiery Language

Many medical terms dealing with fever start with the prefix pyro- or pyr- from the Greek word for “fire.” Fever itself is called pyrexia (pi-RECKS-ee-a). Substances that cause fever are called pyrogens, and medicines that reduce fever are called antipyretics.

The same Greek root has given rise to words outside medicine. A funeral pyre is a consuming blaze used to cremate a body (turn it to ashes). Pyromania is a compulsion to set fires. Pyrotechnics are fireworks. Pyrex is the trade name for a kind of glass used for baking dishes because it can withstand high heat.

A medical professional uses a tympanic thermometer to measure a patient's temperature.

A medical professional uses a tympanic thermometer to measure a patient's temperature.
Image Point Fr/Shutterstock.com.

Mercury thermometers are made out of glass and contain liquid mercury. They come in oral or rectal versions. Either kind can be used in the armpit as well. They are cheaper than digital thermometers, but they take longer to use.

Thermal thermometers are noninvasive and increasingly used in medical settings. With a stroke across the skin of the forehead, the thermal thermometer measures arterial temperature—the heat emitted from the skin over the temporal artery. They are highly accurate and suited to all ages.

Tympanic (tim-PAN-ik) thermometers are a special kind of digital thermometer that is placed into the ear. While the other thermometers take several minutes to give a reading, the tympanic thermometer takes only a few seconds. Ear thermometers are considered inaccurate by medical professionals because the positioning of the probe in the ear canal is inconsistent, resulting in inconsistent readings.

When Should a Doctor Be Consulted?

A doctor should be consulted if a fever is high (over 100° F or 37.7° C), lasts longer than a few days, or is accompanied by other symptoms, such as a rash; pain in the joints, neck, or ears; unusual sleepiness; or a dazed or very sick feeling. For babies under about three months of age, a doctor should be consulted about any fever.

The doctor will try to find and treat the underlying cause of the fever. Antibiotics can cure many bacterial infections, such as those that cause many earaches and sore throats. No medications are available to treat most viral infections.

How Is Fever Treated?

In a basically healthy older child or adult, there usually is no medical reason to treat the fever itself unless it is very high. In fact, lowering the fever with drugs can make it harder to tell if a person is actually getting better or if the drugs are just keeping the fever down. In younger children, though, doctors often treat fevers above 100° F, in part to avoid febrile convulsions. Of course, if a person of any age is very uncomfortable or unable to sleep, even a low fever should be treated to provide relief.

Fever can be lowered by drugs called antipyretics (an-ti-py-RET-iks) that do not require prescriptions. The major ones are acetaminophen * , ibuprofen * , and aspirin. However, aspirin should not be given to children or teenagers with a fever. If children have a viral illness, such as influenza or chickenpox, aspirin makes it more likely that they may get a rare but dangerous illness called Reye's syndrome. This possibility does not exist with their taking acetaminophen or ibuprofen.

Antipyretic medicines are available in pills for adults, chewable tablets for children, and liquid drops for babies. Acetaminophen also comes in suppositories (su-POZ-i-tor-eez), waxy pellets that are inserted into the rectum. They are used for people who cannot take medicine by mouth for whatever reason.

A lukewarm bath can also help lower a high temperature. However, cold water or alcohol rubs can do more harm than good by causing the body to shiver, which just raises body temperature more. In addition to these treatments, it is important for a person with a fever to drink plenty of liquids to avoid dehydration * . In extreme cases, a person in the hospital with a very high fever may be wrapped in a special cooling blanket or immersed in ice water.

How Can Fever Be Prevented?

Many of the diseases that cause fever can be prevented by vaccination * . These include influenza, measles, mumps, German measles (rubella), chickenpox, diphtheria, and typhoid fever. A number of diseases that cause widespread fever in less-developed nations have been eradicated in the United States by good sanitation systems and access to clean water. Still other diseases, such as colds and strep infections, often can be prevented by washing the hands properly before eating and, if possible, by avoiding contact with people who already have these infections.

See also Bacterial Infections • Chickenpox (Varicella) • Common Cold • Dengue Fever • Diphtheria • German Measles (Rubella) • Heat-Related Injuries • Infection • Influenza • Measles (Rubeola) • Mumps • Reye's Syndrome • Rheumatic Fever • Rocky Mountain Spotted Fever • Scarlet Fever • Seizures • Shingles (Herpes Zoster) • Typhoid Fever • Viral Infections • Yellow Fever


Books and Articles

Hamilton, Jennifer L., and Sony P. John. “Evaluation of Fever in Infants and Young Children.” American Family Physician 87 (2013): 254–60. http://www.aafp.org/afp/2013/0215/p254.html (accessed December 7, 2015).


HealthStatus.com , Inc. “When Is a Fever Dangerous?” https://www.healthstatus.com/health_blog/wellness/fever-dangerous/ (accessed December 7, 2015).

HealthyChildren.org . “Fever.” American Academy of Pediatrics. (accessed December 7, 2015).

Medical News Today. “What Is Fever (Pyrexia)? What Causes Fever?” http://www.medicalnewstoday.com/articles/168266.php (accessed December 7, 2015).


Centers for Disease Control and Prevention. 1600 Clifton Rd., Atlanta, GA 30333. Toll-free: 800-232-4636. Website: http://www.cdc.gov (accessed December 7, 2015).

* immune system (im-YOON SIStem) is the system of the body composed of specialized cells and the substances they produce that helps protect the body against disease-causing germs.

* autoimmune disease (awtoh-ih-MYOON) is a disease in which the body's immune system attacks some of the body's own normal tissues and cells.

* rheumatoid arthritis (ROOmah-toyd ar-THRY-tis) is a chronic disease characterized by painful swelling, stiffness, and deformity of the joints.

* lupus (LOO-pus) is a chronic, or long-lasting, disease that causes inflammation of connective tissue, the material that holds together the various structures of the body.

* inflammation (in-fla-MAY-shun) is the body's reaction to irritation, infection, or injury that often involves swelling, pain, redness, and warmth.

* leukemia (loo-KEE-me-uh) is a form of cancer characterized by the body's uncontrolled production of abnormal white blood cells.

* lymphoma (lim-FO-muh) refers to a cancerous tumor of lymphocytes, cells that normally help the body fight infection.

* transfusion (trans-FYOO-zhun) is a procedure in which blood or certain parts of blood, such as specific cells, are given to a person who needs them due to illness or blood loss.

* seizures (SEE-zhurs) are sudden bursts of disorganized electrical activity that interrupt the normal functioning of the brain, often leading to uncontrolled movements in the body and sometimes a temporary change in consciousness.

* Reye's syndrome (RYES SINdrome) is a rare condition that involves inflammation of the liver and brain, and sometimes appears after illnesses such as chickenpox or influenza. It has also been associated with taking aspirin during certain viral infections.

* acetaminophen (uh-see-teh- MIH-noh-fen) is a medication commonly used to reduce fever and relieve pain.

* ibuprofen (eye-bew-PRO-fin) is a nonsteroidal anti-inflammatory drug (NSAID) used to reduce fever and relieve pain or inflammation.

* dehydration (dee-hi-DRAY-shun) is a condition in which the body is depleted of water, usually caused by excessive and unreplaced loss of body fluids, such as through sweating, vomiting, or diarrhea.

* vaccination (vak-sih-NAY-shun), also called immunization, is giving, usually by an injection, a preparation of killed or weakened germs, or a part of a germ or product it produces, to prevent or lessen the severity of the disease caused by that germ.

Disclaimer:   This information is not a tool for self-diagnosis or a substitute for professional care.

(MLA 8th Edition)