I agree there must be a connection, but am clueless beyond that. I have made some observations, though:

1. at diagnosis, our neuro told us that everyone would get PD if we lived long enough (he must subscribe to the "it's a disease of age" theory)...and maybe he is right, yet my grandmother on my dad's side lived to be thirty days shy of 100, and although quite feeble, did not have PD...on my mom's side, grandmother lived to be 96 and although she had "dementia" it did not appear until around year 94, 95. So I am not too sure that it is just a matter of how old you are-look at how many young onsetters there are, including my husband. And look at how many really old folks we have who do NOT have it, including the black fellow in the news during the last election, he was, I think it was, 102 and voting!

2. on the environmental theory, I see that, but did read something recently that made me pause: in India they have been using mucuna to treat PD symptoms for over 4,000 years (6,000 according to one thing I read), all of this was way before we had the chemicals, stress, lifestyle we have now...makes me wonder about how strong the connection between PD and those things is (not that I am discounting that, and we personally live as stress free as one can with two young kids, and grow some of our food, buy organic when we can)...

3. my husband's father, I am pretty sure, also has PD, it is hard to be 100% sure because he will not discuss it and is being treated for more serious heart and lung issues. But I think he does. He is close to 80 but my husband was dx'd at 43. It seems young onsetters have more than a coincidental link with someone in their immediate family with PD. So personally, I believe there is a genetic propensity for this in the first place, but have no idea what the trigger is, or if there even is a trigger.

4. connection between these diseases? They do all affect our brain, but with PD who knows what really happens first? We really are not even sure PD is a "neurological" disease...just now they are making noises that maybe, just maybe, there may be more systems involved, but the neuros don't like that for obvious reasons.