Although normal body temperatures can vary throughout the day (lower in the morning and higher in the afternoon), 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) may be 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 temperatures again give the most accurate reading; ;ear or armpit temperatures are acceptable to screen if there is a fever, but not to give a definitive temperature. For children older than 5, oral temperatures are the main method, while ear and armpit are acceptable for screening. Fever strips are not recommended because those temperature readings have not been found to be as accurate as other methods. Forehead thermometers may not provide as accurate temperature measurements as rectal temperatures.
When someone has a fever, the body raises the normal body temperature (as measured orally) above 37.5°C (99.5°F). A rectal temperature above 38°C (100.4°F) or an underarm temperature above 37.2°C (99°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 with 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 the body 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 and less in the elderly 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 to produce substances called pyrogens. It’s the pyrogens that actually cause the fever.
The body also produces pyrogens in response to viral 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 a fever too.
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 responds 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, so they don’t have the same increase in body temperature.
About 2% to 5% of all children between 3 months and 5 years of age will have a febrile seizure (seizures caused by fever). About one-third of children who have previously had febrile seizures will have at least another febrile seizure. However, these seizures do not appear to cause long-term effects.
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 the symptoms, because the degree of fever doesn’t tell you how sick someone is. If there are symptoms other than fever that are bothersome, a health care professional should be contacted.
Consult a doctor or get immediate medical attention if:
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:
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 elevated white blood cells, and may check the urine or sputum to help determine the cause of the infection. Other tests that may be done include X-rays, cultures of the blood, and ultrasound.
Here are a few things to do for relief until the fever breaks:
Sponge baths with lukewarm water or alcohol are not recommended because they can cause additional shivering and alcohol can be absorbed through the skin.
Since fever protects the body from injury or infection, doctors generally only treat fevers above 102°F (38.9°C) in children, and above 101.3°F (38.5°C) in adults. However, fevers in children less than 6 months old should be reported to a doctor immediately.
Up to about 8 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 to other parts of their body.
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 you how sick the child is.
If a child has a fever and no other symptoms, the fever is not considered dangerously high unless it is higher than 104°F (40°C). Even a fever this high won’t damage the child’s brain or cause any permanent health problems. But very high fevers in children can cause seizures.
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. Aspirin (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 for fever reduction. 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.
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