I saw a close friend decline from a 90%-ile athlete to having trouble getting out of bed within a span of 2 months, and it was heartbreaking. This is someone I looked up to in the gym, went climbing with, went on hikes with. Living in Spain, he did have a hard time getting a proper diagnosis, with doctors eventually settling on ME/CFS. It's been 2 years now and he is slowly on his way back to being functional; went back to his job and he can even do light exercise once a week now.
I am going to ask him if he was prescribed corticosteroids as part of his regime. I do remember during his early tests (I know because I accompanied him) that he did test positive for Mononucleosis antibodies, but he doesn't remember showing any symptoms at that time.