This was brought up earlier and I don't have the energy to backtrack but the point about illnesses being taken as something else initially but later recognized as the diverse manifestations of one illness is astute.
The conditions involved like FM, IBS, TMJ, interstitial cystitis, etc. are in and of themselves diagnoses of exclusion or are unclear diagnostic entities. (I especially am aware of this case in IBS as one of my former classmates specialized in IBS research.) For example, someone with lupus might have joint pains, rashes, seziures, kidney problems. If we didn't know about lupus, we could just as easily say that this person had several co-morbidities (making it up here but for example's sake, we could say this person has arthritis, rosacea, etc. as co-morbidities) when in fact the base illness is lupus and this explains all this person's symptoms.
Focusing on unique aspects of this illness, as Mithriel wrote above me, is extremely important.