https://www.scientificamerican.com/article/are-human-body-temperatures-cooling-down/

It is one of those facts of life that we learn early and don’t forget: normal body temperature is 98.6 degrees Fahrenheit. But a new study in eLife argues that that number is outdated.

The figure was probably accurate in 1851, when German doctor Carl Reinhold August Wunderlich found it to be the average armpit temperature of 25,000 patients. Times have changed, though, according to the recent paper: the average American now seems to run more than a degree F lower.