Parkinson's Disease mostly develops in people after the age of 50, although the disease also affects a small percentage of younger people as well. Approximately, 0.1-0.2% of the population have Parkinson's, but for people over 50, people are ten times more likely to suffer from the disease, with 1-2% of the over 50's having Parkinson's.
Over 4 people million in the world have Parkinson's - more than those affected by Multiple Sclerosis (MS), Muscular Dystrophy, and Lou Gehrig's Disease combined. Over one million people in the United States have Parkinson's.
Current research indicates that Parkinson's Disease may result from a combination of genetic and environmental factors. The environmental factors include exposure to toxic substances, such as pesticides and herbicides.

Because of these causes for Parkinson's, the chances of developing the disease are fairly constant and low – the rates are not particularly increasing or decreasing.