It is, but keep in mind that there are biological markers that are correlated with having chronic fatigue syndrome at p < 0.001 kind of levels.

e.g. if you happen to have blood work from a patient pre-CFS and post-CFS it's very, very common to see a raised T-cell count and an immune response against the Epstein-Barr virus (or some other retrovirus) in the post-CFS that isn't there pre-CFS.

Of course, it's not a great diagnostic tool because you need the patient to have had some blood work just before they got sick. But it does make the "it's all just psychosomatic for everyone" argument a bit unlikely.

None of those biomarkers has been replicated, and stress causes ENV reactivation.

Does EBV reactivation implies fatigue and full mononucleosis symptoms or it just means T-cells count increases ?