While small numbers of cases of Parkinson's have been attributed to specific chemical toxins (such as the herbicide Paraquat) I'm not aware of any documented viral connection to Parkinson's, and most cases don't seem to follow a pattern that would suggest an infectious cause.

And yes, we have plenty to learn about the gut/brain connections. This is my medium article from a year about the possible microbiome connection to ADHD: https://medium.com/invisible-illness/is-my-gut-biome-causing-my-adhd-43f100d5dd8b