Normal body temperature for people. Back in the day doctors and mothers paid a lot more attention to temperature than they do now. As a kid I had to run 100 degrees before Mom would feel I was too sick to go to school. That didn't happen often.
I notice that a fair number of mammals run the same body temp. Stroke a cat, it feels warm, just about the right amount of warm. If the cat 's body temperature was much removed from 98.6 it would feel either cold and clammy, or hot and feverish. Same goes for dogs. And horses. I am guessing that 98.6 is pretty much normal body temperature for all mammals.
I suppose there is a scientific bio chemical reason for this magic temperature, but I don't know what it might be. I don't remember reading anything about it, and I do a fair amount of reading.
High school chemistry taught me that the speed of chemical reactions depends upon temperature. Mammals run on bio chemical reactions, and to stay active when it's cold, they generate heat and keep their bodies warm. An evolutionary trick that reptiles never learned.