--especially from the historical perspective--is that people with many, many different kinds of symptoms were being lumped together under that diagnosis without them showing much commonlity at all on laboratory or other testing, so that the diagnostic category began to be looked at askance in medical circles, in much the same way "fibromyalgia" currently is.

For instance, for a long time, many people thought that one of the hallmarks of the chronic fatigue constellation was a re-activation of Epstein-Barr virus, as evidenced by various antibody titres. But some well-designed research showed that a large number of people who showed such "EBV re-activation" were not experiencing the symptomology of chronic fatigue, while many people WITHOUT such titre evidence were experiencing it. This certainly muddied the waters.

Recent thinking in this realm, by the way, involves a closer look at human herpes viruses 6 and 8 as underlying factors (Epstein Barr is human herpes virus 4--and all human herpes viruses are "immortalized" in the body after initial infection, waiting for an opportunity to reassert).

There is, evidently, SOMETHING going on with peiople who have symptoms of crushing fatigue, muscle and nerve pain, and the like. But there's yet to be a real "smoking gun" type of clinical/laboratory correlation that doctors can point to, and that has kept many from thinking it's a "real" condition. (My suspicion is that we're going to find that environmental toxicity/mitochondrial effects are a big factor here, as we unravel the human genome.)