I was in my late 20s when I got a whiplash injury and my HMO sent me to a "physical therapist" who was really an incompetent chiropractor. He ruined every joint in my spine and I still have some pain from that over 40 years later. My doctor insisted on giving me lots of cortisone injections in my neck. When I told him my pediatrician had said I should never have any more cortisone for the rest of my life the doctor told me it would stay in my spinal column and never absorb into my system. Obviously he lied, but I was naive and believed him.
At about the same time I got mononucleosis. Two of my co-workers also got it near the same time. I was working in the dorms at California School for the Deaf and one of the kids was from Incline Village, but he was healthy so I don't know if there was a connection or not.
After a few months I tested free of the mono and was told I was well. I still had all the symptoms, although my energy level had improved somewhat. I managed to drag myself through work days, but could do nothing else but rest.
After about two years the symptoms got a lot worse again and I had to quit my job because I was too sick to work at all. After several visits I finally talked my doctor into testing me. (He said he'd order the test just to shut me up.) The results showed I did have mono. That doctor told me it's impossible to get mononucleosis more than once so all the earlier tests must just have been laboratory errors. After that all the doctors in the HMO refused to test me again, telling me I wasn't sick and that my problems were just psychosomatic.
Anyone who has CFS or a similar illness will probably say that sounds familiar.