Look into the Wim Hof aka the Iceman. Through dealing with the grief of losing his wife, he created a breathing method to control autoimmune response and voluntarily producing adrenaline. What this means is that he is capable of superhuman feats. He climbed Mount Everest shirtless in shorts, stayed submerged in a bucket of ice for nearly two hours with no change to core body temp (regular person hits hypothermia in less than 20 minutes) and was able to suppress his immune response (fever, increased heart rate, etc.) to introduction of bacteria into his blood stream.

All of this (except Mt. Everest) was done in clinical conditions with doctors and scientists monitoring his body. They have no way to explain his abilities and he has started teaching others his techniques and they are also able to do some of the things he can do e.g. generate adrenaline, suppress autoimmune responses etc.

Check out the Vice video on Wim first to get an overview of him.