The Facts

Although normal body temperatures can vary throughout the day, the average adult normal body temperature when taken by mouth with a thermometer is 37°C (98.6°F). The normal rectal temperature is approximately 0.5°C (1°F) higher than the oral (mouth) temperature, while the temperature under the armpit (axillary) is slightly lower than the oral temperature.

Temperature readings taken rectally are considered more reliable than oral readings, particularly in the case of children and adults who are mouth-breathers. Ear temperature measurements are not accurate in small children and are not recommended for children less than 2 years of age.

Recommendations for temperature measuring techniques vary according to age. For infants and children up to 2 years old, rectal temperatures give the most accurate reading. A thermometer at the armpit can help identify whether or not a fever is present. For children 2 to 5 years old, rectal, ear, or armpit temperatures are acceptable. For children older than 5, oral temperatures are the main method, while ear and armpit are also acceptable. Fever strips are not recommended because those temperature readings have not been found to be as accurate as other methods.

When someone has a fever, the body raises the normal body temperature (as measured orally) above 37.5°C (99.5°F) to try to kill bacteria or viruses in the body. A rectal temperature above 38°C (100.4°F) or an underarm temperature above 37.3°C (99.1°F) is also considered a fever.

Fever is actually the body's natural way of defending itself from invaders like viruses and bacteria, because many of them can't survive in the body due to the high temperature caused by a fever. High body temperatures also signal infection-fighting cells of the immune system such as phagocytes, neutrophils, and lymphocytes to come to the body's defence and help fight off infections. The degree of temperature increase doesn't necessarily correspond to the severity of the illness. The fever response tends to be greater in children than in adults.


Fever can be caused by factors outside or inside the body. Microorganisms, including bacteria and parasites, can produce chemical poisons. Both the microorganism and the poisons cause the white blood cells (called monocytes) to produce substances called pyrogens. It's the pyrogens that actually cause the fever.

The body also produces pyrogens in response to infection, inflammation, cancer, or an allergy. Illnesses in which the body's immune system attacks its own tissues (called an autoimmune disease), such as rheumatoid arthritis, can also cause fever. Too much exercise in hot weather, overexposure to sunlight, or some medications can cause a fever that is a medical emergency. In these situations, get immediate medical attention.

Symptoms and Complications

When the body is fighting an injury or infection, the hypothalamus (a part of the brain) sets the body temperature at a higher level. The body compensates for this by moving blood away from the skin so the amount of heat lost through the skin is reduced.

The muscles might repeatedly contract to keep the body warm, which causes shivering. When the blood that is warmed up to the new temperature reaches the hypothalamus, these symptoms usually stop, and just the fever remains. When the body's thermostat is set back to its normal temperature, it moves the blood back to the skin and excess heat is lost through sweating. Sometimes chills occur when this happens.

The body's temperature may go up and then either return to normal or stay up. Seniors, very young people, and people addicted to alcohol may lose body heat when they're fighting a major infection.

About 3% of all children between 18 months and 3 years of age will have a seizure when they have a high fever. About one-third of children who have previously had febrile seizures (seizures caused by fever) will have another seizure when they have a fever. However, these seizures do not appear to cause long-term effects.

Making the Diagnosis

In most cases, fever can be managed without seeing a doctor. When deciding whether to call the doctor or not, it's better to look at all of your or your child's symptoms, because the degree of fever doesn't tell you how sick someone is.

Consult a doctor or get immediate medical attention if:

  • a child below the age of 6 months has a fever
  • a child has a fever that lasts more than 3 days
  • a child or adult with a history of cancer, AIDS, or other serious illness such as heart disease or diabetes has a fever
  • a child or adult has a fever after returning from international travel
  • fever is accompanied by:
    • behaviour changes
    • excessive fussiness or irritability
    • headache, stiff neck, confusion
    • localized pain, redness, or swelling
    • persistent vomiting or diarrhea
    • skin rash
    • shaking chills and burning or pain with urination
    • shortness of breath and cough
    • unresponsiveness or limpness

When a child has a fever, trust your parental instincts. Take your child to the doctor or get immediate medical attention if there is something about their behaviour or appearance that worries you.

If you see your doctor about a fever, they will take a medical history – asking about symptoms, prior diseases, medications, and recent travels. Usually, it's easy to find a specific cause for a fever. If a specific cause can't be identified, additional tests may be performed.

Your doctor may want to know the following about a fever:

  • how long the fever has lasted
  • if the fever is worsening or getting better
  • if there are chills or other symptoms (e.g., headache, cough)
  • if the fever is going up and down
  • if it came on suddenly
  • what other additional symptoms you may be experiencing
  • if you have started any new medications recently

After asking detailed questions, your doctor will also perform a physical examination to look for an infection or signs of a disease. Your doctor may check the blood for white blood cells, and may check the urine or sputum. Other tests that may be done include X-rays and ultrasound.

Treatment and Prevention

Here are a few things to do for relief until the fever breaks:

  • drink plenty of fluids (e.g., water, juices, broth or oral hydration solution) to compensate for fluid loss from sweating
  • get plenty of rest
  • make sure there are no extra blankets or clothing on the body, to help lower the body temperature (to prevent shivering and a subsequent rise in body temperature, do not remove all clothing)

Sponge baths with lukewarm water or alcohol are not recommended because they can cause shivering and alcohol can be absorbed through the skin.

A fever causes the body to use more oxygen. Thus, people who have difficulty getting more oxygen into their blood, such as heart and lung patients, should be treated for a fever as soon as one develops.

Antipyretics, which are medications that fight fever, are used to help people with a fever to feel more comfortable. Acetaminophen* and ibuprofen are frequently used. ASA (acetylsalicylic acid) is given only to adults because it can cause Reye's syndrome, a disease that causes liver and brain damage in children. Acetaminophen or ibuprofen is generally given to children who are uncomfortable because of a fever. These are considered very safe and effective when used as recommended. Since fever is part of the body's natural defense against infection, the goal of using these medications is to improve overall comfort, not to reach a "normal" body temperature.

When using these medications, the dose should be based on the child's weight rather than age. To ensure an accurate dose is given, use a medication cup or oral syringe to give the liquid forms of the medication. Keep all medication out of the reach of children. If a bacterial infection is the suspected cause of a fever, your doctor will likely prescribe antibiotics.

If a medication is causing the fever, the medication will be stopped and other treatments may be used as well. If heat exhaustion is causing a fever, immediate medical attention is necessary, as the body temperature needs to be reduced quickly and medications typically used to reduce fever are not effective.

*All medications have both common (generic) and brand names. The brand name is what a specific manufacturer calls the product (e.g., Tylenol®). The common name is the medical name for the medication (e.g., acetaminophen). A medication may have many brand names, but only one common name. This article lists medications by their common names. For information on a given medication, check our Drug Information database. For more information on brand names, speak with your doctor or pharmacist.