Overview
Although normal body temperatures can vary throughout the day, the average adult normal body temperature when taken by mouth with a thermometer is 37.8°C, or 98.6°F. The normal rectal temperature is approximately 0.5ºC (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 should not be used in children under three 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 or ear temperatures are acceptable and 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 above 37.8°C (100°F) to try to kill bacteria or viruses in the body. 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 defend 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 adults.
Causes
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, hormonal problems, or some medications can cause fever, as well.
Symptoms
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. People with alcoholism, seniors, and very young people 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 with a high fever. About one-third of children with a history of febrile seizures (seizures caused by fever) will have another seizure when they have a fever. However, these seizures are not associated with long-term effects. Medications can be used to help prevent febrile seizures.
Treatment
When body temperature is greater than 40°C (104°F), and is prolonged, here are a few things someone can do for relief until the fever breaks:
- drink plenty of fluids (water) to compensate for fluid loss from sweating
- get plenty of rest
- remove extra blankets and clothing so heat can leave the body and help lower the body temperature (but don't take off all the clothes as that can can lead to shivering and make body temperature rise again)
- sponge the body with lukewarm water, which may help reduce fever (although it can cause children to feel uncomfortable)
Since fever protects the body from injury or infection, doctors generally only treat fevers above 39°C (102°F) in children, and above 38.5°C (101°F) in adults. However, fevers in children less than two months old should be reported to a doctor immediately. If a child between two months and three years has a fever over 39°C (102.2°F), or over 38.5°C (101°F) that has lasted 24 hours, contact a doctor immediately. Between three and six years of age, children can have a fever for 48 hours, and in school-age children 72 hours, before it needs medical attention.
Up to about eight weeks of age, a fever can be a sign of a serious underlying disease, since newborns don't have other symptoms when they have an infection. They also can't fight infections as well as older children, so their infections are more likely to spread. In general, doctors will treat a fever if the rectal temperature is above 39°C (102°F) or if the oral temperature is above 38.5°C (101°F).
Children's fevers are higher and more frequent than those of adults. When deciding whether to call the doctor or not, it's better to look at all of a child's symptoms because the degree of fever doesn't tell someone how sick the child is. If a child has a fever and no other symptoms, the fever is not considered high unless it is higher than 40°C (104°F). Even a fever this high won't damage the child's brain or cause any permanent health problems. However, very high fevers in children can cause seizures. Although not very common, children who have had febrile seizures (seizures caused by fever) should get treatment if there is any fever at all.
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 lower body temperature. Acetaminophen* and ibuprofen are frequently used. ASA 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 are generally given to children for fever reduction and are considered very safe and effective. If a bacterial infection is the suspected cause of a fever, your doctor will likely prescribe antibiotics.
*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 more information on brand names, speak with your doctor or pharmacist.
Other tips:
In most cases, fever can be managed without seeing a doctor. However, if there are symptoms other than fever that are bothersome, a health care professional should be contacted. Consult a doctor if:
- a child below the age of three months has a temperature of 38ºC (100.4°F)
- a child has a fever of 39°C (102.2°F) or greater
- an adult has a fever of greater than 40.5°C (104.9°F)
- a child or adult with a history of cancer, AIDS, or other serious illness such as heart disease or diabetes has a fever
- fever is accompanied by:
- headache, stiff neck, confusion
- shaking chills, burning or pain with urination
- shortness of breath and cough
- localized pain, redness, or swelling
- the fever remains high after an hour or two despite self-treatment
- the fever has not gone away in 2 days
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
- if the fever is going up and down
- if it came on suddenly
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.
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