personally, i think brain cells can die from many causes, including from pathogens. i also think if pathogens were the primary reason for pd it would occur much earlier and in infants who haven't developed a strong immune system.
plus there would be obvious clusters.

antibiotics are fed to chickens since by doing so they pt on weight faster, antibiotics have affects on normal cells, not just bacteria. it is very difficult to get a new antibiotic approved due to side affects and the most powerful antibiotics have strong side affects, often neuropathy.

is pd more prevalent in immune system diseases such as aids? cancer is. that's a highly studied population.

do they have a higher incidence of pd in the humid south east or on the olympic peninsula of wash state cuz fungi sure live year round there and in greater diversity? how about in indonesia or other tropical areas? a higher incidence would stick out like a sore thumb.

i think we get pd mainly after middle age mainly because of following possibilities:
1. repair mechanism breaks down with age after which toxins have more affect
2. gradual buildup of alpha-s
3. maybe just born with fewer dopamine producing cells
4. like with mohammad ali, blows to the head killed cells and eventually the normal 1-2% loss of dopamine cells resulted in pd. we have such a large surplus.

i don't accept the premise that 50year olds have a weakened immune system such that a bacteria can suddenly take hold. my impression is that pd starts years before it is diagnosed, possibly in the 30's, but we have so many extra neurons it doesn't manifest itself until 50's.