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.