Just so that it isn't too easy, consider the possibility that there are at least two "stages" to PD, each having multiple paths crossing it to a variant of PD.

That's not the best presentation of an idea that I have ever generated, but let's take neuroinflammation as an example. We know a lot about that one. If the fetus is exposed to bacterial toxins at critical times of development, then it is changed in ways that increase the odds in favor of PD. There is already a ton of complexity in that one statement. Which bacteria? Several different ones. What development windows? Multiples. What changes? Fewer dopaminergic neurons in the SN. A hyperreactive immune response by the microglia that leads to neuronal death. All in that one sentence.

Once born we can still fall prey to the immune system's neuroinflammatory response by coming down with infections. Which ones? Influenza. Nocardia. Salmonella. More? And I suspect that the timing matters. The flu at a time when our stress system is overloaded is going to differ from when we are rested.

All that can lead to us having this loaded gun of neuroinflammation in our head. But once that first stage is in place, what pulls the trigger? A dozen different things. Pestcides. Herbicides. Mercury. Manganese. A bad tooth. A UTI. etc.

I suspect that the mitochondrial branch of the family is just as complicated, too.

I think that Modern Medicine has met its match here.