The U.S. Navy version of the Red Cross flag, flown per the Geneva Convention on naval hospital ships instead of the commission pennant, and also used for some naval onshore establishments.  In areas of the middle east, a version with a red crescent is often used instead.  For more information, see here, w:Emblems of the International Red Cross and Red Crescent Movement, and Navy document NTP 13(B), section 1721.
Own work
Carl Lindberg