About 1.7 million Americans suffer a traumatic brain injury (TBI) every year, about three-quarters of them mild TBIs, or concussions. Yet, while they affect so many people, TBIs received little medical-research funding until brain injuries from the wars in Iraq and Afghanistan — often caused by shock waves from explosions — began to mount in recent years. Nearly 20 percent of veterans deployed in those wars have returned with a TBI, often severe enough to require intense physical, psychological and emotional aid. Since 2007 Congress has poured millions of dollars into TBI research, but most patients still cannot pay for the expensive rehabilitative services that severe brain injuries require. Meanwhile, researchers have found that even a series of mild TBIs, such as those suffered by many football players, substantially raises the risk of severe dementia and depression.