In 1989 I had some blood work done which checked for Epstein-Barr (it wasn't present) for reasons that made sense at the time. Then I got ME/CFS a few weeks later, and blood work showed that it was present this time.
That's not a slam-dunk for Epstein-Barr as a cause, of course, but I thought it was interesting enough to have the data that I talked to my immunologist about cowriting a paper on it.
She demurred, saying that quite a few such papers (where patients by chance had had before-and-after tests) had been published and there wasn't any value in adding another one, since it was pretty common to see the same pattern of Epstein-Barr virus not being there beforehand, and then appearing after in blood work after the onset of symptoms.
Fast forward to 2023 and now there's a paper saying that ME/CFS might have a post-viral infection origin.