Fibro and CFS are definitely medical conditions with symptom lists and exclusion criteria. There are recognisable signs like a reduction of mitochondria, changes in white cells and many other things which I don't keep in mind these days. I do know our research showed that the average number of symptoms of a person with CFS was 22 at any one time, though they can change and come and go.
CFS has been a recognised medical condition for over 50 years, though some doctors still "don't believe in it", as if that would make the condition go away and the people healthy. A doctor's "belief" in this situation does nothing for their patients.
Etiology is variable, though it could be summarised as an "immune assault", with many coming down with it after an infection, often, but not exclusively Epstein-Barr. It has also been diagnosed after vaccinations, pregnancy and other attacks on the immune system. I have also heard of cases occurring after car crashes, but I would expect to have found an underlying infection occurring simultaneously if I had been reviewing those cases.
Most doctors treat Fibro and CFS with pain killers and antidepressants where I live, though I've heard of other things in the US. I wouldn't choose any medication, but would seek healing, rather than treatment if I was back in that situation again.