A not-so offhand hypothesis

The reason our military personnel in Iraq and Afghanistan are having such high rates of mental illness is due to the weather conditions.

They go over there and live in the sunny, dry desert or the cold,dry mountains and get head infections. And by the time they get back, the immumologically susceptible ones are chronically ill.

I'm guessing they put them on low-fat diets too...

Update- look what I just found:
Report: Study finds link between PTSD, compromised immune systems in veterans