One difficult issue with ME is to explain the cases that don't have a clear infectious origin, or at least not as clear as the 80% of cases that do. I've long strongly suspected that it was likely due to mild cases that seem so benign as to be forgotten and a slower onset. I'm in that camp. There was that weird episode a few months before it all went to hell, some really weird insomnia and odd dizziness that came and went. I couldn't sleep at all for a night. Mostly that. It was weird but otherwise unremarkable since it mostly went away.
I've posted a few times with examples of testimonies that fit that pattern, people initially either barely ill or who have "recovered" from acute Covid, to the point of being able to function normally for weeks, sometimes months, before the typical ME course began. Sometimes there is little remarkable improvement in-between. Sometimes there is, creating a separation in the timeline that makes a remarkable event seem unimportant.
This trend continues unabated. I am pretty certain this explains most of those cases. An initial mild infection that resolves enough to feel back to normal, with a slower onset of ME symptoms happening later. Enough that people would normally not even think about it, given how usual it is to have such mild illness.
The sheer number of those cases, people who were "recovered" for weeks, months, before they developed the symptoms may be the missing link, if not for all cases certainly a solid % of them. Explaining a large % of a remaining 20% or so gets us very close to a full deck.
Of course there are still some cases that are harder to explain, from accidents, even pregnancies, or surgeries. But can anyone name a more iconic duo than pathogens and opportunistic infection, taking advantage of a weakened body state, especially if latent pathogens, of which there are many, play some role?
This is one example of many, I don't keep rigorous track, I couldn't even if I wanted to. But threads like this one are pretty representative of that pattern:
Am I a long-hauler?. A pattern that would normally be missed entirely. It's so unremarkable otherwise. This is the stuff that completely breaks standard medical practice. It takes special care to notice it. Or a pandemic. Whichever comes first, I guess.