Finally, there is currently a bill in Congress to start a national PD and MS Disease registry that will help to answer some of these questions. See PAN Action Alerts to ask your Represenatatives and Senators to cosponsor these bills.
House bill is HR 1362
http://capwiz.com/pan/issues/alert/?alertid=13800281

Senate bill is S1273
http://capwiz.com/pan/issues/alert/?alertid=13572346
Summary of bill
"The National MS and Parkinson's Disease Registries Act will establish separate, permanent, and coordinated MS and Parkinson's disease national registries at the Agency for Toxic Substance and Disease Registry (ATSDR) at the Centers for Disease Control and Prevention (CDC). As you know, we do not have accurate information on how many Americans are battling Parkinson's disease and MS and why. The registries will determine incidence and prevalence of the diseases and lay a foundation for better evaluating and understanding many factors, including geographic clusters of diagnoses, variances in gender ratio, changes in health care practices, and changes in disease burden and population over time. To develop the registries, the CDC will use a methodology that relies on de-identified information from existing databases"