red cross (n.)

early 15c., national emblem of England (St. George's Cross), also the badge of the Order of the Temple. Hence red-cross knight, one bearing such a marking on shield or crest. In 17c., a red cross was the mark placed on the doors of London houses inflected with the plague. Red Cross (in Muslim lands, red crescent) adopted as a symbol of ambulance service 1864 by the Geneva Conference.