In Last Child in the Woods, we learned about how modern life has created a new epidemic in children, something Louv called Nature Deficit Disorder. The Nature Principle is packed with information on studies and experiences suggesting that adults, too, can have Nature Deficit Disorder. There is a lot of evidence that many of our common psychological ailments (depression, schizophrenia, etc) are either caused by lack of nature exposure or can be corrected or eased by nature therapy.
We probably haven't done enough research on nature therapy for specific diseases, but I definitely buy into the concept of nature as a healing force. When my husband, Nathan, worked in Glacier National Park and I would take a week away from my highly stressful software engineering job to hike and be completely disconnected from the grid, my whole outlook on life changed. I thought that was just because I was on vacation, but there is a big difference between how I feel after a week in a European city and how I feel after a week in the backcountry. I think Louv is on to something. Check out The Nature Principle if you're interested in learning more!
Case in "point": I am one happy hiker in this photo!
Pitamakin Point - Glacier National Park - July 2007