I had a concussion when I was about 15....It was the only obvious health difference I could think of between me and my twin.
Neck issues cropped up sometime after ME/CFS.....nothing major - X-rays don't show anything major but at times quite painful.
I've had too many injuries to list over the years, but that said - I also wonder about an incident that happened just months before my final big decline.
I was helping at a community garden and stood up too quickly underneath the edge of a deck. (The water spigot was underneath and hard to access - even for a short person).
Anyway I stood up straight underneath, thinking I had cleared the edge but hadn't. I clocked myself in the very top of my head, jarring my spine with enough force to fall on the ground.
Because I'd had so many previous injuries I went to the ER to get checked out. No real findings other than the MRI showed some unusual deterioration in my neck. Doctor commented on it, but hey- no concussion so I was declared fine.
Funny thing is that immediately after (days) the injury I felt like it had reset something and my posture was actually better! Then that summer I was noticing I couldn't walk a mile without extreme fatigue. Super odd.
Then that fall I got a major flu that was mistakenly diagnosed as pneumonia. I was sick for over a month.... And then I never got better! Game over.
In my case it seems impossible to pinpoint any ah-ha moment. I seem to have every damned possible cause under the sun! Tracing back to childhood, with phases of sickness and phases of being in great shape and super athletic.... With random bouts of bizzaro illnesses that were hard to kick.