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Condition Info > F > Fever
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(Elevated Temperature · High Temperature)

In this condition factsheet:

The Facts on Fever

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.

Causes of Fever

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 of Fever

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.



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