While I am not certain what current medicine is saying when I studied Parkinson's disease in 2008 there were many causes essentially boiling down to this:
Genetics played a role in that it increases the chance that an individual would develop the disease if exposed to the right enviromental factors such as drugs.
While I can't remember the exact case, in the 90s there was an isolated outbreak of Parkinson's disease on the West Coast. Several people developed symptoms overnight due to something that was added to the weed they had recently smoked.
They volunteered for dopamine research (injections into the brain) and had great success in creating motion, but without any control because their genetic material still couldnt' copy itself correctly.
If you are concerned about your genetic chances, you should seek aid froma genetic counselor, most hospitals can point you in the right direction.