Comment by Lazare

I mean, the S in CFS stands for "syndrome", which is "a set of medical signs and symptoms which are correlated with each others [...] When a syndrome is paired with a definite cause this becomes a disease." (From wikipedia.)

So I mean, yeah, that literally does mean "we don't know what this is, and we don't know what's causing it, so we're dumping everything that looks like it in a bucket while we do more research". But that doesn't mean it's not a real thing; it means that we don't know what it is or what's causing it (and that it may well not be a single thing at all).

That's pretty different than saying "it's not a thing at all".