Earlier quoted context omitted.
Chronic fatigue can be caused by undiagnosed organic illnesses. Personal anecdote: I had CFS for several years, and when the diagnosis (an easily treatable genetic condition) was eventually made, and treatment taken, I was better within a few months.
Yes, but by definition it isn't CFS if that is the case, as CFS is a diagnosis of exclusion at present. It's certainly possible that CFS is caused by some as-yet-unidentified organic illness. However the science doesn't seem to point in that direction (as the stress system seems to be centrally implicated), and it also doesn't fit with the experiences of patients like myself who have fully recovered.
Certainly, stress would exacerbate any symptoms of fatigue.