This is a Parkinson’s map that I found. https://www.targetmap.com/viewer.aspx?reportId=53520
It implies sunshine might be a factor. It also could be genetics, or maybe infectious.
For myself, I was was having muscle problems, and one symptom I had was shaking after using my muscles. I didn’t have Parkinson’s, but maybe it could have progressed to some sort of more permanent muscle shake. Vitamin D significantly helped me get rid of the shake. But, I think Parkinson’s is something from the brain, and my problem felt like a problem in the muscle or nerve transmission, but I never thought about it in regards to something in the brain.
Vitamin D has been, and is still being studied in relation to Multiple Sclerosis. MS clusters in the upper Midwest of the US, and it’s high in numbers in northern and Western Europe. That could be D, or could be genetics. The upper Midwest is full of Germans and Vikings.