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A fever is an elevated body temperature. It is defined in medicine as "a state of elevated core temperature, which is often, but not necessarily, part of the defensive responses of multicellular organisms (host) to the invasion of live microorganisms or inanimate matter recognized as pathogenic (something which causes illness) or alien by the host."[1] In everyday usage, however, a fever is usually defined as a rise in body temperature above the normal range, usually by one or two °C. What is considered the normal range in body temperature (typically 37°C or 98.6°F) is open to debate.[2] Recently, scientists have defined the normal body temperature using a thermometer in the mouth as 37.2°C (98.9°) in the morning and 37.7°C (99.9°F) overall in healthy adults aged 40 years or younger.[3]

A fever isn't an illness itself, but an indication that something out of the ordinary is going on in the body. A fever isn't necessarily bad. In fact, fevers seem to play a key role in helping the body fight off a number of infections.

Clinical thermometer showing a temperature of 38.7°C (101.6°F). Source: Wikimedia Commons.


Other Names

  • Febrile illness
  • Pyrexia


A fever occurs when the body temperature rises above its normal range. What is normal for one individual may be a little higher or lower than the average temperature of 98.6 °F (37 °C). But a rectal temperature higher than 100.4 °F (38 °C) is usually always considered a fever. A rectal temperature reading is generally 1 °F higher than an oral reading.

Even when an individual is healthy, the body temperature varies throughout the day. It is lower in the morning and higher in the late afternoon and evening. The "normal" temperature taken by mouth can range from about 97 °F (36.1 °C) to 99.9 °F (37.7 °C).[3]

Normal body temperature is regulated by several areas in the brain and nervous system, including the hypothalamus, an area at the base of the brain that acts as a thermostat for the body.[2] In a healthy individual, anything that results in a rise in body temperature, for example exercising, causes the regulatory system to activate body mechanisms, such as sweating, to reduce the temperature back to normal. In a situation where fever occurs, substances in the body called pyrogens, discussed below, alter the activity of the regulatory system so that the body's temperature rises to a higher degree before cooling mechanisms are activated. Therefore a person with a fever will not sweat, even though body temperature is higher than normal, until a fever "breaks" (starts to resolve). When the temperature regulating system is exposed to pyrogens, the body increases heat production mainly by shivering. In newborns, non-shivering mechanisms are involved in increased heat production.

In many cases, the rise in body temperature that occurs with a fever can be viewed as a beneficial response in the body's attempt to fight off infecting microorganisms. Many bacteria thrive in an environment of around 98.6 °F (37 °C), but don't reproduce as well at a higher temperature of only one or two degrees. Thus, the mechanisms of fever may have developed as an evolutionary response to infecting microbes.[4]


Pyrogens are substances that can cause a fever. They can be produced by one's own cells (endogenous pyrogens) or can result from exposure to a microorganism (exogenous pyrogens). Endogenous pyrogens are chemicals produced by specific cells in the body, mainly macrophages and other phagocytic cells (cells that engulf invading microbes), in response to an invading microbe or to exogenous pyrogens. Some endogenous pyrogens include interleukin-1, tumor necrosis factor, interleukin-6, and interferon gamma.[2] The mechanism by which endogenous pyrogens actually induce fever is not fully understood. The pathways appear to be complex and involve a number of other chemicals such as prostaglandin E2.[2]

Exogenous pyrogens are components of microorganisms that trigger an inflammatory response, which includes a fever. Endotoxin or LPS, a component of Gram negative bacteria, is a potent pyrogen.

Associated symptoms

Because a fever can occur with many different conditions, other signs and symptoms can often help identify the cause. Depending on what's causing the fever, additional fever symptoms may include:

  • Sweating
  • Shivering
  • Headache
  • Muscle aches
  • Lack of appetite
  • Dehydration
  • General weakness

Very high fevers, from 103 °F (39.4 °C) to 106 °F (41.1 °C) and higher, may cause the following:

  • Hallucinations
  • Confusion
  • Irritability
  • Convulsions
  • Fever-induced seizures (febrile seizures). Febrile seizures are often triggered by a fever from a common childhood illness such as roseola, a viral infection that causes a high fever, swollen glands and a rash.

A fever almost never rises above 105.8 °F (41 °C) to 107.6 °F (42 °C) in humans.[5]

Fever can be a side effect of some medications such as antibiotics and drugs used to treat high blood pressure or seizures. Some infants and children develop fevers after receiving routine immunizations, such as the diphtheria, tetanus and acellular pertussis (DTaP) or pneumococcal vaccines.

Sometimes it's not possible to identify the cause of a fever. If the body temperature is higher than 100.9 °F (38.3 °C) for more than three weeks and the doctor isn't able to find the cause after extensive evaluation, the diagnosis may be fever of unknown origin.[6] [7] In most cases, though, the cause of the fever can be found and treated.

What a Fever Could Mean

A fever usually means the body is responding to a viral or bacterial infection. Sometimes heat exhaustion, extreme sunburn or certain inflammatory conditions such as temporal arteritis (inflammation of an artery in the head) may trigger fever as well. In rare instances, a malignant tumor or some forms of kidney cancer may cause a fever.


Most fevers go away in a relatively short time (usually within a few days). Not all fevers need treatment with medications.[8] It's possible for fever medications to have side effects, especially in young children. Therefore, consultation with a physician may be necessary before administering fever medication to small children.

The doctor will likely diagnose the cause of the fever based on other symptoms and a physical exam. Sometimes additional tests are necessary to confirm a diagnosis.

If a low-grade fever persists for three weeks or more, without other symptoms, the doctor may recommend a variety of tests to help find the cause. These may include blood tests and X-rays.

Medical treatment depends on the cause of the fever. Antibiotics may be prescribed for bacterial infections, such as pneumonia or strep throat. For viral infections, including stomach infection (gastroenteritis) and mononucleosis, the best treatment is often rest and plenty of fluids.

The doctor may also make a recommendation about using over-the-counter medications, such as acetaminophen (paracetamol, e.g. Tylenol) or ibuprofen (e.g. Advil, Motrin) to lower a high fever. Adults may also use aspirin. However aspirin should not be used to treat a fever in children as it may trigger a rare, but potentially fatal, disorder known as Reye syndrome.

It is not advisable to try to lower the body temperature before consulting a physician (i.e. by taking a cold shower). Doing so may only prolong the illness or mask underlying symptoms and make it harder to determine the cause.

Some experts believe that aggressively treating a fever may actually interfere with the body's ability to fight an infection. That's because the viruses that cause colds and other respiratory infections thrive at cool temperatures. A slight fever can help the body get rid of such viruses.[4]


Because the body loses more water with a fever, drink plenty of fluids to avoid dehydration. Water is a good choice for adults, but the best liquid for a sick child under one year of age is an oral rehydration solution (such as Pedialyte). These solutions contain water and salts in specific proportions to replenish both fluids and electrolytes in children.

People with fever should get plenty of rest. Other recommendations for both older children and adults include the following:

For fever below 102 °F (38.9 °C)

  • Don't use any medication for a fever in this range unless advised by a doctor.
  • Don't give children aspirin because of the risk of Reye syndrome. Instead, dress in comfortable, light clothing and try bathing in lukewarm water. At bedtime, use a sheet or light blanket.

For fever between 102 °F (38.9 °C) and 104 °F (40 °C)

  • Take acetaminophen or ibuprofen according to the label instructions or as recommended by a doctor (adults may use aspirin instead).
  • Acetaminophen is available in liquid, chewable, and suppository forms for children, but it's often easiest to give medications in liquid form. For a small child, use a syringe with measurements on the side and a bulb on the tip. Gently squirt the medicine in the back corners of the child's mouth.
  • Avoid taking or administering too much medication. High doses of acetaminophen, or taking the drug for a long time, may cause liver or kidney damage, and an overdoses can be fatal. Side effects of aspirin and other nonsteroidal anti-inflammatory drugs, such as ibuprofen, include stomach pain, bleeding, and ulcers.

For fever above 104 °F (40 °C)

  • Use a five- to ten-minute sponge bath of lukewarm water to try to bring the high temperature down. A sponge bath is most likely to help if it's used shortly after a dosage of acetaminophen or ibuprofen is taken, so that the medication can work to keep the fever down after the bath takes effect.

If a child shivers in the bath, stop the bath, dry the child and wait. Shivering actually raises the body's internal temperature (shaking muscles generate heat). If the fever doesn't get better or the child has a febrile seizure, seek immediate medical care.

When to seek medical attention

Fevers by themselves may not be a cause for alarm—or a reason to call a doctor. Yet there are some circumstances when medical advice should be obtained for infants, children, or adult as described below.

For Infants

An unexplained fever is a greater cause for concern in infants and children than in adults. Consult a pediatrician (children's doctor) if the baby:

  • is younger than three months of age and has a rectal temperature of 100.4 °F (38 °C) or higher.
  • is older than three months of age and has a temperature of 102 °F (38.8 °C) or higher.
  • has a fever and unexplained irritability, such as marked crying.
  • has a fever and seems lethargic and unresponsive. In infants and children younger than two years of age, these may be signs of meningitis, an infection and inflammation of the membranes and fluid surrounding the brain and spinal cord.
  • is a newborn and has a lower than normal temperature—less than 97 °F (36.1 &degC). Very young babies may not regulate their body temperature well when they are ill and may become cold rather than hot.

For Children

Consult a pediatrician if the child:

  • is listless or irritable, vomits repeatedly, has a severe headache or stomachache, or has any other symptoms causing significant discomfort.
  • has a fever after being left in a very hot car. Seek medical care immediately.
  • is younger than two years of age and has a fever that persists for longer than one day, or longer than three days in a child two years of age or older.
  • has special medical circumstances, such as immune system problems or a pre-existing illness.

Sometimes, older children can have a lower-than-normal temperature. This can happen to older children with severe damage to the nervous system, children with a life-threatening bacterial infection in the blood (sepsis), and children with a weakened immune system.

For Adults

Consult a physician about a fever if:

  • the body temperature is more than 103 °F (39.4 °C).
  • the duration of the fever is more than three days.

In addition, consult a physician immediately if any of these signs and symptoms accompany a fever:

  • Severe headache.
  • Severe swelling of the throat.
  • Unusual skin rash, especially if the rash gets rapidly worse.
  • Unusual eye sensitivity to bright light.
  • Stiff neck and pain when bending the head forward.
  • Mental confusion.
  • Persistent vomiting.
  • Difficulty breathing or chest pain.
  • Extreme listlessness or irritability.
  • Abdominal pain or pain when urinating.
  • Any other unexplained signs or symptoms.

Taking a Temperature

To check body temperature, several types of thermometers are available, including electronic thermometers and ear (tympanic) thermometers.

Thermometers with digital readouts and those that take the temperature quickly from the ear canal are especially useful for young children and older adults. Because the contents of glass mercury thermometers harm both humans and the environment, they are no longer recommended.

A temperature is normally taking by placing an oral thermometer in the mouth, or a rectal thermometer in the rectum, and leaving it in place for three to five minutes. The thermometer is then removed and the temperature read. Both types of thermometers should be cleaned after use with alcohol or soap and warm water.

Although it's not the most accurate way to take a temperature, an oral thermometer may be used for an armpit (axillary) reading as follows:

  • Place the thermometer in the armpit with arms crossed over the chest.
  • Wait four to five minutes. The axillary temperature is about 1 degree Fahrenheit lower than an oral temperature.

Use a rectal thermometer for infants as follows:

  • Place a dab of petroleum jelly on the bulb of the thermometer.
  • Lay the baby on his or her tummy.
  • Carefully insert the bulb one-half inch to one inch into the baby's rectum. Hold the bulb and the baby still for three minutes.


A rapid rise or fall in temperature may cause a fever-induced seizure febrile seizure in children younger than age 5. Although alarming, the majority of febrile seizures cause no lasting effects.

If a seizure occurs:

  • Lay the child on his or her side.
  • Remove any sharp objects that are near the child
  • Loosen tight clothing and hold the child to prevent injury
  • Don't place anything in the child's mouth or try to stop the seizure
  • Although most seizures stop on their own, call for emergency medical assistance if a seizure lasts longer than 10 minutes.
  • Try to note which part of the child's body begins to shake first. This can help the doctor understand the cause of the seizure.


The best way to prevent fevers is to reduce exposure to infectious diseases. One of the most effective ways to do that is also one of the simplest, frequent hand washing.

Frequent washing of the hands, especially before eating, after using the toilet, after spending time in a crowded public place, or after petting animals, can minimize exposure to bacteria or viruses that can result in fever. Try not to touch the nose, mouth or eyes, these are the main way viral infections are transmitted.

Wash the hands vigorously, covering both the front and back of each hand with soap, and rinsing thoroughly under running water. Carry hand-washing towelettes for times when access to soap and water is not available.


  1. IUPS Thermal Commission. Glossary of terms for thermal physiology. 2nd ed. Pfugers Arch. 1987;410:567-87. Reference
  2. 2.0 2.1 2.2 2.3 Mackowiak PA. Concepts of fever. Arch Intern Med. 1998 Sep 28;158(17):1870-81. Abstract | Full Text | PDF
  3. 3.0 3.1 Mackowiak PA, Wasserman SS, Levine MM. A critical appraisal of 98.6 degrees F, the upper limit of the normal body temperature, and other legacies of Carl Reinhold August Wunderlich. JAMA. 1992 Sep 23-30;268(12):1578-80. Abstract
  4. 4.0 4.1 Mackowiak PA. Fever: blessing or curse? A unifying hypothesis. Ann Intern Med. 1994 Jun 15;120(12):1037-40. Abstract | Full Text
  5. Mackowiak PA, Boulant JA. Fever's glass ceiling. Clin Infect Dis. 1996 Mar;22(3):525-36. Abstract
  6. Gaeta GB, Fusco FM, Nardiello S. Fever of unknown origin: a systematic review of the literature for 1995-2004. Nucl Med Commun. 2006 Mar;27(3):205-11. Abstract
  7. Knockaert DC, Vanderschueren S, Blockmans D. Fever of unknown origin in adults: 40 years on. J Intern Med. 2003 Mar;253(3):263-75. Abstract | Full Text | PDF
  8. Klein NC, Cunha BA.Treatment of fever. Infect Dis Clin North Am. 1996 Mar;10(1):211-6. Abstract
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