A fever is a rise in body temperature to greater than 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. In turn, the thermostat 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
  • inducing sweating, which cools the body through evaporation

A fever occurs when the body’s thermostat resets at a higher temperature, which primarily happens 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, which leaves the surface and extremities cold.

Once the body reaches the higher temperature, 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. When this happens, 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. Certain chemicals in the immune system react with the fever-inducing agent and trigger the resetting of the thermostat.

These immune system chemicals also increase the production of cells that fight off the invading bacteria or viruses. Higher temperatures also inhibit the growth of some bacteria and speed up the chemical reactions that help the body’s cells repair themselves. Changes in blood circulation may cause the heart rate to increase, which speeds the arrival of white blood cells to the sites of infection.

Causes & symptoms

Fevers are primarily caused by viral or bacterial infections, such as pneumonia or influenza. However, other conditions can induce a fever, including these:
  • allergic reactions
  • autoimmune diseases
  • trauma, such as breaking a bone
  • cancer
  • excessive exposure to the sun
  • intense exercise
  • hormonal imbalances
  • certain drugs
  • 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. These pyrogens trigger the resetting of the thermostat. In other circumstances, an uncontrolled release of pyrogens may occur when the immune system overreacts due to an allergic reaction or becomes damaged due to an autoimmune disease.

A stroke or tumor can damage the hypothalamus, causing the body’s thermostat to malfunction. Excessive exposure to the sun or intense exercise 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.

A recent study showed that most parents have misconceptions about fever and view it as a disease rather than a symptom. How long a fever lasts and how high it may go depend on several factors, including its cause and the patient’s age and 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 peak 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 oral and rectal thermometers made of glass and mercury, and more sophisticated electronic ones that can be inserted in the ear. 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 temperatures taken by placing an oral thermometer under their armpits. 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 physician can make a diagnosis by checking for accompanying symptoms and by reviewing the patient’s medical history, any recent trips he or she has taken, what he or she may have ingested, or any illnesses he or she has been exposed to. Blood tests hold additional clues.

Antibodies in the blood point to the presence of an infectious agent, which can be verified by growing 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.


Often, doctors must remind patients, especially parents, not to “overtreat” low fevers but to remember that they are symptoms of an underlying disease or condition. Alternative therapies for treatment of fever focus not only on reducing fever but also on boosting the immune function to help the body fight infections more effectively. They include nutritional therapy, herbal therapy and traditional Chinese medicine.

Nutritional therapy

Naturopaths often recommend that patients take high doses of vitamin C to ward off diseases and prevent fever. In addition to vitamin C, other antioxidants such as vitamin A and zinc also boost the immune function. Naturopaths may also suggest reducing sugar intake (even fruit juices) because sugar depresses the immune system. To replace fluid that is lost during fever, patients are advised to drink vegetable juices and eat soups.

Herbal therapy

Western herbalists use tea preparations containing herbs such as bupleurum root or boneset to reduce fever. Mild herbs such as peppermint, elderflower, or yarrow can provide comfort to the child who has a mild fever. Others believe in sweating a fever out, literally. They often recommend that patients take hot baths to induce sweating. This helps induce or increase fever, which is believed to help the body get rid of infections.

Chinese medicine (TCM) offers many herbs and formulas for fevers. There are many distinct kinds of fevers, also called heat syndromes. For example, an excess- heat syndrome is characterized by a high fever, great thirst, and lots of sweating. Deficiency heat syndrome is characterized by a low-grade fever with afternoon fevers or night sweats. For excess heat, herbs that are dispersing and cold in nature are used.

For chronic and low-grade fevers, herbs that tonify the yin (cooling aspect) are used as well as herbs that get rid of heat. There are even herbs such as bupleurum root (called Chai Hu in TCM) that are used for intermittent fevers or conditions alternating between fever and chills. Alternating fevers and chills occur in malaria, conditions connected to AIDS, chronic fatigue syndrome, and Epstein- Barr virus. The individual pattern should be diagnosed by a trained practitioner.


Patients can reduce feverish symptoms by inhaling essential oils of camphor, eucalyptus, lemon, and hyssop. These oils can also be mixed with an unscented body lotion or a vegetable oil for aromatherapy massage.


Homeopathic doctors may prescribe herbal remedies based on the patient’s overall personality profile as well as specific symptoms.

Allopathic treatment

Physicians agree that the most effective treatment for a fever is to address its underlying cause. Also, because a fever helps the immune system fight infection, some clinicians suggest it 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, and ibuprofen. 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. Sponging a child or infant with tepid (lukewarm) water can also help reduce mild fevers.

A fever requires emergency treatment under the following circumstances:
  • Newborn (three months or younger) with a fever above 100.5°F (38°C).
  • Infant or child with a fever above 103°F (39.4°C). A very high fever in a small child can trigger seizures (febrile seizures) and therefore should be treated immediately.
  • Fever accompanied by severe headache, neck stiffness, mental confusion, or severe swelling of the throat. A fever accompanied by these symptoms can indicate the presence of a serious infection, such as meningitis, and should be brought to the immediate attention of a physician.

Expected results

Most fevers caused by infection end as soon as the immune system rids the body of the pathogen. Most fevers 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.