The Marine Corpsman, i.e. Medic, is a marine of the S.M.C. who specializes in medical care, both on and off the battlefield. If you have medic with you and your are hurt, go near and interact and he will heal you. If the medic is with you in some level, try to keep them alive as they are sometimes your objective, if they die the mission is over. However usually in these situations the medic will be tougher than they should be.