Research just published in Annals of Neurology
, 2015, suggests that Parkinson's Disease starts in the gastrointestinal tract and spreads to the brain, through the vagus nerve. This is based on information from a large epidemiological study which found that people that had their vagus nerve severed were significantly less likely to develop Parkinson's. Researchers noted that those with the vagus nerve only partially severed did not have reduced risk.
Aarhus University. "Parkinson's disease may begin in the gut: Parkinson's disease begins in the gastrointestinal tract, large study indicates." ScienceDaily. ScienceDaily, 23 June 2015. <www.sciencedaily.com/releases/2015/06/150623103609.htm>.