Philip K. Dick 
Piper in the Woods [PDF ebook] 

Earth doctor Henry Harris is tasked with the responsibility of examining the men who return from missions across the galaxy. Travelers returning from Asteroid Y-3 begin to exhibit strange psychosomatic symptoms — specifically, they seem to believe that they are plants. When this anomaly becomes an epidemic, Dr. Harris travels to Asteroid Y-3, hoping to get to the bottom of things. Are the men suffering from some kind of mass delusion, or have they stumbled across a hidden truth?