As a combat engineer in Iraq, Jeremy was supposed to find roadside bombs. They found him instead. Within 72 hours of each other, two improvised explosive devices (IEDs) went off within 15 feet of this father in his late 20s. The first set of blast waves, a moving wall of highly compressed air that emanates from an explosion, knocked him out briefly. The second left him dazed for about 30 minutes and produced ringing in his ears that disappeared within a week. These detonations did not visibly injure Jeremy (not his real name)—but he was never the same.