The April 15, 2013 Boston Marathon bombing killed three people and injured 264 – 20 critically – but every patient who was transported to a hospital survived the tragedy. One reason is that Boston is a “medical mecca,” with an unusually high number of teaching hospitals and trauma centers. But another crucial reason is that the Boston medical community worked hard over the past decade to build emergency-preparedness programs and to practice for mass casualty events in exercises and drills.