It is hard to see PD as arising from any single cause. The patterns just aren't right. For example, if the cause is mercury in vaccines, then we would expect to find a "bulge" in the data with most PWP having been vzccinated and a similar lack of cases among those who have not.

Geographical distribution has produced a very few clues such as more cases in the Dakotas and fewer in India, but all tantalize while keeping their treasures hidden.

Socio-economic factors don't offer any "eureka" moments, either. I, personally, believe that the changes wrought by the Industrial Revolution played a role as wheat varieties were standardized, sugar became cheap, artificial light and shift work disrupted circadian rhythms, dust in the workplace became a given, the family support system was destroyed, etc.

We must be looking for multiple factors working together. Maybe mercury does play a role, but only if stress produced levels of hormones are just so. A large number of possible combinations yielding a common outcome could account for a lot.

Maybe we can gain something by leapfrogging over the question of cause and considering the processes. Inflammation of the brain, for example, becomes more common as we age, is worsened by infection, rotenone, mercury, stress, etc.

I am currently pursuing the role of blood pressure in the brain. It can surge quickly and fade away just as fast, leaving the BBB in tatters. I have found that my worst freezing is when my BP is high. Tissues balloon and recede as it fluctuates.