Body temperature is a measure of the body's ability to generate and get rid of heat. The body is very good at keeping its temperature within a narrow, safe range in spite of large variations in temperatures outside the body.

When a person is too hot, the blood vessels in the skin expand (dilate) to carry the excess heat to the skin's surface. The person may begin to sweat, and as the sweat evaporates it helps cool the body. When a person is too cold, the blood vessels contract so that blood flow to the skin is reduced to preserve body heat. The person may start shivering, which is an involuntary, rapid contraction of the muscles. This extra muscle activity helps generate more heat. Under normal conditions, these processes keep the body temperature well regulated.

A normal body temperature is usually referred to as an oral temperature of 98.6°F (37°C), but that is an average of normal body temperatures. Many healthy people normally have a temperature 1°F (0.56°C) or more above or below 98.6°F (37°C). Also, normal body temperature varies by 0.5°F (0.28°C) to 1°F (0.56°C) throughout the day, depending on a person's activity level, the time of day, and other variables, (for instance, whether a woman is ovulating or having her menstrual period).

A rectal or ear (tympanic membrane) temperature reading is 0.5°F (0.28°C) to 1°F (0.56°C) higher than an oral temperature reading. In adults, a fever is sometimes defined as an oral temperature above 100°F (37.78°C) or a rectal or ear temperature above 101°F (38.33°C). A child has a fever when his or her rectal temperature is 100.4°F (38°C) or higher.

A fever may occur as a reaction to:

• Infection. This is a common cause of a fever. Infections may affect the whole body or a specific body part (localized infection).
• Medications such as antibiotics, narcotics, barbiturates, antihistamines, and many others. These are so-called drug fevers. Some medications, such as antibiotics, raise the body temperature directly; others, including nonsteroidal anti-inflammatory drugs (NSAIDS) and phenothiazines (such as Compazine, Mellaril, or Thorazine), interfere with the body's ability to adjust its temperature when other factors cause the temperature to rise.
• Severe trauma or injury, such as a heart attack, stroke, heat exhaustion or heat stroke, or burns.
• Other medical conditions, such as arthritis, an overactive thyroid (hyperthyroidism), and even some cancers (such as leukemia, Hodgkin's disease, and liver and lung cancer).

An abnormally low body temperature (hypothermia) can be serious, even life-threatening. Low body temperature may occur due to cold exposure, shock, alcohol or drug use, or certain metabolic disorders, such as diabetes or an underactive thyroid (hypothyroidism). Oddly, a low body temperature may also occur in certain infections, particularly in newborns, older adults, or people who are sickly. Certain overwhelming infections, such as Gram-negative sepsis, may also cause an abnormally low body temperature.

Body temperature is measured with a thermometer. There are several types of thermometers, each with its own advantages and disadvantages as discussed below. Thermometers for home use are calibrated in either degrees Fahrenheit (°F) or degrees Celsius (°C), depending on the custom of the region. Temperatures in the United States are often measured in degrees Fahrenheit, but the standard in most other countries is degrees Celsius.

Body Temperature
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Rectal and ear (tympanic membrane) temperatures normally run about 0.5°F (0.28°C) to 1°F (0.56°C) higher than an oral temperature, while an armpit temperature is usually about 0.5°F (0.28°C) to 1°F (0.56°C) lower than an oral temperature. If your oral temperature is 99°F (37.22°C), your rectal or ear temperature may be about 100°F (37.78°C) and your armpit temperature about 98°F (36.67°C). When you tell your doctor about your temperature measurement, be sure to mention whether it was taken in the mouth, rectum, armpit, or ear.

When you have a fever, keeping a daily record of your own temperature with multiple recordings will help you and your doctor define what is causing the fever and how it can be treated. The pattern of a fever can help suggest the reason for the body's increased temperature. For example, a fever that stays high for several days may indicate a flu or pneumonia. A temperature that rises and falls within a day may suggest an infection in a specific body part (abscess). A fever that comes and goes every few days may suggest certain kinds of infection, such as malaria.

In most healthy people, a temperature below 103°F (39.44°C) does not in itself require treatment other than drinking plenty of fluids to replace losses from evaporation. Taking acetaminophen (such as Tylenol) or ibuprofen (such as Advil or Motrin) may help you feel more comfortable. Do not give aspirin to anyone under the age of 20. Use of aspirin in people under age 20 has been associated with a serious condition called Reye's syndrome.

In most instances, a fever can be considered a part of your body's natural defense against infection. However, in some circumstances you may need to call a health professional about a high fever.