we must have pretty good defenses against environmental toxins, etc. since with identical twins, no greater chance other twin will get pd, slight familial chance and you just don't see clusters even though within in a community it's likely everyone gets some exposure. so seems you have to have just the right combination of "insults" to your system, maybe add in a bad flu, concussion, just 1 episode of a chemical exposure.

personally, i can think of things that happened to me that didn't happen to my siblings, concussions playing football, getting chicken pox in my 30's, taking a systemic medicine for toenail fungus, working in a research lab with a lot of organic solvents, worked in ag research around experimental chemicals, the list goes on.
maybe just one tainted meal?