Any member of the Armed Forces that is either constantly at a medical facility, constantly complaining of being sick or injured, or both. In general, these are the people who work harder trying to get out of work than they would by doing the initial tasks assigned to them.
SGT: Hey, where is PFC Blake?
CPL: He is at medical.
SGT: WHAT!? AGAIN!? That guy is a freaking sick bay commando. He is worthless. I guess we will all have to carry the load for him again, as always.