Some things that you brought up above are very haunting. What if destruction of the motor areas in the brain can become an "epidemic" as was seen in 1918 with the "leftovers" of the spanish flu?
What if cerain nations develop a "Weapon" to cause PD much like we did to kill off our American Indian Brothers and sisters, when we gave them blankets filled with contractable diseases?
This could be in the form of a "Silent" agent which would destroy people within certain boundaries and not effect others.
I don't want to go here, it's too scary, but i'm sure that others have thought of it before. cs