I was diagnosed with Chronic Fatigue Immune Dysfunction Syndrome by a practicing Doctor of Internal Medicine (not by a "wellness clinic" style doctor).
Primary causal factor in medically diagnosed cases is—without exception—stress.
Anyone who knew me then can vouch for it. I was in a bad way. But, if you knew me now you'd never guess I'd ever had it. In fact, it went away the week after my disassociation letter was accepted. Even at the worst of my (suicidal) depression I was not as physically wiped out as when I was ill with CFS.
I have had no more joint pains, the constant headache I lived with since age 15 was gone, no more tension through the shoulders, no more roving pains, no more outbreaks of chicken pox or cases of mononucleosis ... gone.