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.