But you can lose a lot of energy reinventing the wheel. In order to save some of that, I am going to unabashedly lay out some answers. That it is IMHO should go without saying because it is all opinion because nobody knows. So, here is "PD for Dummies"-

1- PD is a "junk diagnosis" assigned when your doc is either stumped, busy, or incompetent. Certain criteria leads to its selection over other such catch-alls. Tremor, for example. Slow? Stiff? You got PD baby.Unless, of course, you cure yourself later....
2- Because of the manner of diagnosis, PD has several sub-sets of patients. YOPD. SOPD. Tremor dominant. Akinetic. Fast progressing. Slow. and so on.
3- We have to paint with a broad brush to account for what we see. It will not be found that microbe "A" nor virus "B" nor toxin "C" causes PD.
4- PD is caused by inflammation, for the most part. Inflammation is caused by many secondary agents. Viruses. Microbes. Toxins. Mercury. Manganese. High fructose corn syrup. Flu shots. Stress.
5- Stress? Yes, stress causes inflammation. But inflammation also causes stress. Welcome to the world of the feedback loop. Ever been around when a musician laid a microphone too close to his PA? That ear splitting shriek started as a whisper that was amplified and fed back in again and again. That is the power of a feedback loop. Stress and inflammation do a dance that can destroy neurons.
6- The destruction comes from the unintended effects of cytokines and hormones. The first are the messengers of your immune system, especially those defenders called the microglia. They get excited and forget to calm down. This initiates the first phase of PD. The SN has one of the highest concentrations of microglia in the CNS. So the SN takes the biggest hit. Microglia become more excitable aswe age. So PD is thought of as an old man's disease. But some of us are exposed to early life encounters that prime our microglia to over react. We get Young Onset. All this starts the process.
7- Once the immune phase gets started, we are increasingly vulnerable to the endocrine phase. Stress. Chronic, everyday stress and its chemical cousins from the adrenal glands - cortisol and adrenaline. More dead neurons.
8- But the hormones and cytokines are more than toxins, they are also neuroactive. They affect the CNS as surely as dopamine. Get a urinary tract infection and ignore it? Your PD can consume you in days because of your own cytokines.

Notice how little mention there is of neurology here? Oneof the biggest hurdles we face is that PD falls on the cracks where several different disciplines meet. And they don't talk to one another.

More of this (with citations) here.