In 1918, there was an epidemic/pandemic (not sure of the difference), but a lot of people got encephalitis. They later all developed PD. Why?
Because encephalitis causes the BBB to widen its pores.

"Viruses are another possible environmental trigger for PD. People who developed encephalopathy after a 1918 influenza epidemic were later stricken with severe, progressive Parkinson's-like symptoms."
http://www.ninds.nih.gov/disorders/p...ns_disease.htm

Just editing this to add that whilst the BBB is there to keep out certain molecules, it also stops a number of drugs that the medics want to use on PD. To get these drugs in to the brain, they either use a "trojan horse" method, or they deliberately open the pores of the BBB with a compound that widens the pores, and add the drug.
In the above report on encephalitis, the BBB is only open for the duration of the disease. This pushes forward the day they reach the threshold of 80% of neurons put out of action, and PD starts. So are the medical people piling up problems for their patients by temporarily opening the BBB to let their drugs in, plus a rush of toxins that cause the person to get PD at some later date???

Ron