The following has been shown in peer reviewed research (i.e. I'm not theorizing):
1) Exposure to bacterial toxins in the womb has three effects
a) lower density of dopaminergic neurons in the substantia nigra
b) changes to endocrine system that makes the stress response poorly controlled
c) changes to immune system that increases system inflammation and also sets up a hyper-reaction in the brain itself to future toxins of the same type

2) b and c above both tend to open the BBB particularly when stress or illness are a factor

3) the bacterial toxin (LPS) is always present in the body so when the BBB leaks LPS entere the brain and the hyper-reaction in c above is triggered and neurons are destroyed by the immune system

4) throughout all this the immune system is producing chemicals (cytokines) and so is the endocrine (corticoids) - both enter the brain and damage nerve tissue. Both are also neuroactive and interfere with brain function

after a few decades of this PD is born