When it came to things that added up to throw me into severe ME/CFS in the 1990's, I had a full list. I'm willing to bet a lot of us are like that, plus we have something in our DNA that hurts more than it helps.
That is why it is so hard to pinpoint one thing that tips the scale. It's different for each of us even though we have similar things on our lists.
family history of fibromyalgia (father
signs of Sjogrens Syndrome that started as a youngster
-> lots of radiation exposure as a young vet tech working for a creep of a vet who didn't care about my health
bad fight-or-flight experience with a horse rolling over my head
lots of chemical exposure in veterinary school (formaldehyde, organophosphates..
-> radioactive Iodine treatment for thyroid issues
EBV, Rocky Mtn. Spotted Fever
high-stress job with way too many hours
black mold exposure (the tipping point, for me
-----> FM, SS, ME/CFS, MCS
and now...PFOS exposure in my drinking water for the past 10+ years
Isn't life on the edge GRAND?