I suspect this may true of more of us than we currently realize. We may have had ME for years (maybe all our lives), but it wasn't recognized until something sufficiently damaging knocked us over the edge into disability.
I often think about the history of HIV and some of the similarities to our history. For years the thinking was that the disease was AIDS, which was recognized when patients developed serious infections from pathogens ordinarily controlled by the human immune system. All AIDS patients were very, very ill people. Once they discovered the cause, HIV, they realized that the majority of patients with the disease were
not those severe AIDS patients, but people who appear healthy or nearly so. The AIDS patients were just the easily-identified severe tip of the iceberg.
The same could be true for us. What we see as "real ME" with PEM, neuroimmune manifestations, and disability (50% reduction in activity level) may be only the severe tip of the iceberg. The illness itself may exist in people who currently don't have the symptom set we think of as ME. They may be walking timebombs, just waiting to encounter the wrong trigger -- a pathogen, a vaccine, who knows? -- that sets off the cascade that spirals down into ME. Or maybe it's just a matter of time for some people as their body systems degrade slowly from the disease without the need for the straw-that-broke-the-camel's-back single pathogen.
One idea I have is that our illness may be at bottom an immune disorder that's been in place a long time, either as a genetic abnormality or an unknown pathogen (like HIV was) that has been in our bodies a long time. The multiple pathogens identified in PWME, none of them with enough frequency to be considered the cause of the illness, could be secondary infections. As in HIV, those secondary infections would be highly problematic, but not causal. They may be what's causing most of our symptoms (which is why our symptoms differ somewhat-- different secondary infections) so treatment is critical, but that doesn't get at the root cause of the illness. Again, like HIV.
My daughter had multiple hits before she finally went down for the count, so to speak. If we hadn't been aware of ME after the first one (because unlike her, I didn't mostly recover from that one), we might have claimed any of those hits as The Cause of her ME -- the weird extremely sudden-onset flu-like illness we both had, the pre-college vaccine boosters that dramaticly increased her ME symptoms, or the H1N1 that left her unable to finish the semester. My guess is that none of them is actually the root cause of her ME, they just moved her condition from apparently healthy to clearly very ill.
Of course, I could be completely wrong.

This is just one of a number of my random speculations about ME.