Fever is an abnormally high 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.
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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-THAL-a-mus), a small part of the brain that also helps control hunger, thirst, pleasure, and pain. The thermostat, called the thermoregulatory (ther-mo-REG-u-la-tor-ee) center, normally keeps the body's temperature hovering around 98.6 degrees Fahrenheit (F) (37 degrees Centigrade).
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, 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 around 98.6 degrees F, the body's heating and cooling systems may keep the temperature at 100 to 102 degrees 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. Many doctors, however, say that a temperature of more than 99 or 100 degrees F (37.2 or 37.8 degrees C) should be considered a fever. A temperature of 104 degrees F or higher could be considered a high fever.
Ups and Downs
A person's temperature normally varies each day by about 1 degree F (0.6 degrees 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 normal. 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 the lower level.
Fever of Unknown Origin
Sometimes a person has a fever that lasts for two or 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, a cause eventually is found. The most common causes are infectious diseases. Fever of unknown origin is particularly common in people infected with HIV, the human immunodeficiency virus that causes AIDS.
Sometimes a person's temperature can rise for a different reason. Hyperthermia (hy-per-THER-me-a) occurs if the heat outside is too much for the body's cooling system to handle, making body temperature rise. The most severe cases of hyperthermia tend to occur in people who can not sweat as much as normal, 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 (ek-SOJ-enus) 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 an autoimmune disease * , such as rheumatoid arthritis or lupus
- has inflammation * anywhere in the body
- has cancer, such as leukemia or lymphoma
- was a given a blood transfusion * that is not compatible with the person's own blood type
- has a reaction to a medication
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, for instance, if their immune system is weak or damaged. In reality, 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 it happens to everyone many times in the course of a lifetime. Young children are particularly likely to get bacterial and viral infections that cause fever, such as strep throat and ear infections. Sometimes minor viral infections cause high fevers in children, while illnesses that are more serious cause milder fevers. People of all ages get fever.
There is some evidence that fever 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 helps people fight off infections in real life. It could turn out that fever helps in certain cases but not in others.
* Immune system (im-YOON SIS-tem) is the body system made up of organs and cells that defend the body against infection or disease.
* autoimmune disease (aw-to-i-MYOON disease) is a disease resulting from an immune system reaction against the body's own tissues or proteins.
* Inflammation (in-fla-MAY-shun) is an immune system reaction to an injury, irritation, or infection, It often includes swelling, pain, warmth, and redness.
* transfusion (trans-FYOO-zhun) is the transfer of blood or blood products directly into a person's bloodstream.
The Name Is Familiar
Many infectious diseases are named for the major symptom of fever. Most of those listed below lead to fevers of about 102 to 104 degrees F (39 to 40 degrees C). Dengue fever, Lassa fever, and yellow fever are caused by viruses. The others are caused by bacteria.
- Dengue (DENG-e)fever causes sudden high fever, headache, extreme tiredness, severe joint and muscle pain, swollen lymph nodes, and a rash. It is spread by mosquitoes.
- Lassa (LAH-sa) fever causes fever, headache, dry cough, back pain, vomiting, diarrhea, sore throat, and facial swelling. It is spread by rats and from person to person.
- Q fever causes sudden high fever, severe headache, and chills. It is spread by farm animals and insects.
Fever often can help in another way, however. It can be an important sign that a person is sick. Its movements up and down can indicate whether a person is getting better or worse.
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. This 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.
Children under age 5 can have a different problem if their temperature rises quickly. They may experience a kind of seizure called a febrile convulsion (FEB-ryl kon-VUL-shun). 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 very upsetting and frightening. They also can lead to injury; for example, if a child falls.
Extremely high temperatures of around 107 degrees F or higher can do permanent brain damage at any age if they last for a long time. Temperatures that high usually are caused by hyperthermia, not by fever from an illness.
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 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 to see how warm it feels, this 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. Three kinds of thermometers can be used: digital, mercury, or tympanic.
Digital thermometers, usually used in medical offices and hospitals as well as at home, are electronic. They can take an oral temperature when placed under the tongue, a rectal temperature when placed into the rectum, or an axillary (AK-si-lar-y) temperature when placed in the armpit. In general, rectal temperatures are about 1 degree F higher than oral ones.
Mercury thermometers, which used to be the only kind available, 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.
- Rheumatic (roo-MAT-ik) fever causes painful, swollen joints, fever, and heart murmurs (abnormal heart sounds). It is caused by the same bacterium that causes strep throat.
- Rat-bite fever causes sudden chills, fever, headache, vomiting, back pain, a rash on the hands and feet, and temporary arthritis (joint inflammation). It is spread by rats and mice.
- Relapsing (re-LAPS-ing) fever causes sudden chills and high fever, fast heartbeat, severe headache, vomiting, muscle pain, and sometimes mental confusion. Symptoms can recur several times. It is spread by ticks and lice.
- Rocky Mountain spotted fever causes fever, headache, skin ulcers (open sores), and a rash. It is spread by ticks.
- Scarlet fever causes high fever, sore throat, flushed cheeks, and a rash, especially in children. It is caused by the same bacterium that causes strep throat.
- Typhoid (TY-foid) fever causes fever along with abdominal pain, headache, and extreme fatigue. It is spread by food and water that contain Salmonella bacteria.
- Yellow fever causes sudden fever, slow pulse, nausea, vomiting, constipation, muscle pains, liver failure, and severe fatigue. It is spread by mosquitoes.
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. However, tympanic thermometers are more expensive and can be inaccurate if placed improperly in the ear.
When Should a Doctor Be Consulted?
A doctor should be consulted if a fever is high, 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 3 months old, 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. There are no medications to treat most viral infections.
How Is Fever Treated?
In a basically healthy adult or older child, 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 of 100 or 101 degrees 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 can 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 (a-seet-a-MIN-oh-fen), ibuprofen (i-byoo-PRO-fen), and aspirin. However, aspirin should not be given to children with a fever. If children have a viral illness, such as influenza or chickenpox, aspirin makes it likelier that they may get a rare but dangerous illness called Reye's syndrome * . This does not happen with 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 some reason.
A lukewarm bath also can 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.
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 (turn to ashes) a body. Pyromania is a compulsion to set fires. Pyrotechnics are fireworks. Pyrex is the trade name for a kind of glass used in baking pans because it can withstand high heat.
* Reye's syndrome (RYZE SIN-drome) is a rare and sometimes fatal disease that causes vomiting, confusion, and coma. It occurs mainly in children, usually after a viral infection such as influenza or chickenpox.
* dehydration (de-hy-DRAY-shun) is a condition caused by the loss of fluids from the body faster than they can be replaced. Babies, small children, and the elderly may become dehydrated faster than older children and adults.
* vaccination (vak-si-NAY-shun) means giving a person a vaccine to protect against a particular disease. A vaccine is a preparation of a weakened or killed germ or of part of the germ's structure. It stimulates the immune system to fight the germ but does not cause infection itself.
How Can Fever Be Prevented?
Many of the diseases that cause fever can be prevented by vaccination * . These include influenza, measles, mumps, rubella (German measles), chickenpox, diphtheria, and typhoid fever. A number of other diseases that cause widespread fever in poorer nations are prevented 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 those infections.
German Measles (Rubella)
Rocky Mountain Spotted Fever
American College of Emergency Physicians, P.O. Box 619911, Dallas, TX
75261-9911. An organization of physicians that provides information
about fever on its website.