To be honest, I don't believe some virus or bacteria is causing PD. I do believe a viral or bacterial infection could trigger PD, but I don't think a virus is causing the whole PD process. But of course, I can't prove this. This is up to the scientists to prove it.

But if a virus or bacteria would be the cause for PD, then why majority of PD patients have late onset PD ? And those with early onset PD have some genes that gives them a high probability of getting PD ? If a virus or bacteria would cause PD, then anyone could get it no matter what age (and even genes) they have. Also, when scientists converted skin cells into neurons, these neurons also had PD; but no bacteria or virus is causing it.

I tend to believe the common story a bit more; the story that some external events can trigger PD. And a bacterial or viral infection can be such an external effect that triggers PD. I heard that doctors have a much higher probability of getting PD than the normal population. Don't know whether that is true.

Anyway, the question you are asking is really a tricky one and I don't think anyone can answer it. Not even scientists know were PD comes from.