No direct impact caused Paul McQuigg’s brain injury in Iraq three years ago. And no wound from the incident visibly explains why Mr. McQuigg, now an office manager at a California Marine base, can get lost in his own neighborhood or arrive at the grocery store having forgotten why he left home. But his blast injury — concussive brain trauma caused by an explosion’s invisible force waves — is no less real to him than a missing limb is to other veterans. Just how real could become clearer after he dies, when doctors slice up his brain to examine any...