Why the Human Body Temperature Is 98.6 Degrees??

I mean 98.6? Why not 98.0? 98.5?

Normal human body temperature, also known as normothermia or euthermia, is a concept that depends upon the place in the body at which the measurement is made, and the time of day and level of activity of the person. There is no single number that represents a normal or healthy temperature for all people under all circumstances using any place of measurement.

Different parts of the body have different temperatures. Rectal and vaginal measurements, or measurements taken directly inside the body cavity, are typically slightly higher than oral measurements, and oral measurements are somewhat higher than skin temperature. The commonly accepted average core body temperature (taken internally) is 37.0 °C (98.6 °F). The typical oral (under the tongue) measurement is slightly cooler, at 36.8±0.7 °C, or 98.2±1.3 °F. In Russia and former Soviet countries, the commonly quoted value is 36.6 °C (97.9 °F), based on an armpit (axillary) reading. Although some people think of these numbers as representing the normal temperature, a wide range of temperatures has been found in healthy people. In samples of normal adult men and women, the observed range for oral temperature is 33.2–38.2 °C (92–101 °F), for rectal it is 34.4–37.8 °C (94–100 °F), for the tympanic cavity it is 35.4–37.8 °C (96–100 °F) and for axillary it is 35.5–37.0 °C (96–99 °F).

Variations in body temperature are part of the circadian rhythm (see at 4:30 and 19:00).The time of day and other circumstances also affects the body's temperature. The core body temperature of an individual tends to have the lowest value in the second half of the sleep cycle; the lowest point, called the nadir, is one of the primary markers for circadian rhythms. The body temperature also changes when a person is hungry, sleepy, or cold.