but more so if combined with other factors. Prions, for example, could indeed be a factor.

But it is a mistake to think in terms of a "cause" for PD in that it is almost certain that it can be "caused" by multiple combinations of factors. My combination will be different than, say, Paula's. But the syndrome that we know as PD is more or less the same because the intermediate processes are the same, i.e. inflammation, for example. One of the problems we face is the single cause mindset has us directing scarce research funds into searching for a single cure/single cause entity. It's like looking for the guy who robbed the bank when it was a gang of ten.