My friend and colleague, Dr. Jennifer Rosenberg, a family physician in Coolidge Corner, was attending to her patient when we heard two explosions ? the second a few moments after the first. The medical tent was already packed with runners lying on cots for dehydration or hypothermia. We heard a calm voice from the center of the tent, belonging to John Andersen, one of our medical coordinators. "Stay calm, and stay with your patients," he said, "while we figure out what's happening." A few minutes later, physicians, nurses, and coordinators were called to the site of the explosion; EMS ran to the scene and returned with stories about dismembered limbs and pools of blood. Immediately, the medical tent was transformed into a trauma unit.