The original observation about teachers and nurses has a simple answer if one looks from an inflammatory viewpoint. One who has a sensitized immune system will react more violently to encounters with bacteria and viruses. This violent reaction is out of proportion to the threat due to the hypervigilance of the microglia. Even separated from the site of an injury by the length of the body and the blood brain barrier, the microglia keep things at a rolling boil of damaging cytokines.

Nurses and teachers are constantly exposed to microbes that trigger the immune response of the microglia. Day in, day out throughout their working life, there is near constant stimulation of the destructive merry-go-round and it spins fastest in the SN.

It isn't that a mysterious virus creeps in and causes PD. The problem is the reaction of our microglia. A lot of things can start the ball rolling and keep it doing so. Those invaders of yore cannot defeat the host's defenders but their presence can revive the memory of old battles. That is what fuels PD and teachers and nurses inhabit a sea of those reminders. Their microglia never rest.