Buffy already knew about CFS

I've started watching Buffy the Vampire Slayer again and 30 minutes in, in the first episode, Cordelia is telling some friends about her bedridden mom whom she suspects to suffer from CFS, whereas her doctor suspects the Epstein-Barr virus.

It's been decades since I've seen the show, but it will probably turn out later to be some kind of demonic influence. If only it were that simple.