I have made a number of posts over the years pointing out apparent associations between environmental factors and variations in the prevalence of PD across the US.

The most notable of these concerned the fungal infection histoplasmosis.

This post links PD to fungal infections in general through relative humidity.

Compare the map produced by Willis et al. of the prevalence of PD in the US:

http://www.ncbi.nlm.nih.gov/pmc/arti...395/figure/F1/

With the map showing the mean annual relative humidity across the US:

http://www.eldoradocountyweather.com...-humidity.html

The match is far from perfect, but both maps show a high zone running north and east from southern Texas, and a smaller high zone in the Pacific North West.

A causal mechanism for the apparent association is given by Arundel et al. [1]:

"Most species of fungi cannot grow unless the relative humidity exceeds 60%."

Although not conclusive in itself, it adds weight to the argument that fungal infections play a role in the pathogenesis of PD.

Reference

[1] "Indirect health effects of relative humidity in indoor environments."
A V Arundel, E M Sterling, J H Biggin, and T D Sterling
Environ Health Perspect. 1986 Mar; 65: 351–361.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1474709/

John