Although normal body temperatures can vary throughout the day, the average adult normal body temperature when taken by mouth with a thermometer is 98.6°F (37°C). The normal rectal temperature is approximately 1°F (0.5°C) 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 under 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 or ear temperatures are acceptable. Taking oral temperatures is the main method for children older than 5, 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 100°F (37.8°C). A rectal temperature above 100.4°F (38°C) or an underarm temperature of 99°F (37.2°C) 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 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.
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, too.