This was converted from 37.0 Celsius.
In Carl August Wunderlich's 1871 "On the Temperature in Diseases" he devotes chapter 5 to "On the Temerature in Health". There is much here on the variation under sundry circumstances but I'll quote a relevant paragraph (page 82): "The untrustworthiness of the observations of healthy temperature, owing to the difficulty of excluding previously existing slight, or (although latent) serious disturbances of health in the subjects of experiment, and the impossibility of sharply severing pathological effects from physiological ones, prevent our positively determining the *range of temperature in healthy human beings*; yet we may accept, as not far from absolute truth (supported as it is by the numerous observations we have the opportunity of making in convalescence), the statement that the *range of normal temperature in the axilla* is from 97.25º Fahr. (36.25º C.) to 99.5º Fahr. (37.5º C.) *and that the mean normal temperature* = 98.6º F. (37º C.)." Note the mixed use of degree of accuracy and (in this instance) the predominance of the Fahrenheit scale. The work is available from Google as a free ebook.