Philip K. Dick 
Piper in the Woods 

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?