A corpsman was a rating used in the United States Navy for an enlisted crewman whose duties were comparable to that of a field medic. They were primarily responsible for providing first aid and care for the wounded in the absence of medical support personnel.
When Pavel Chekov was captured aboard the aircraft carrier USS Enterprise in 1986, where it was believed that he was a "Russkie" spy, the Starfleet commander attempted to escape FBI custody by running. Chekov made it as far as the ship's flight deck when he fell over the side and landed unconscious on the wharf. When the pursuing Marine sergeant arrived at the scene, he called out that there was a "Man down!" and they needed to "Get a corpsman over here." (Star Trek IV: The Voyage Home)