Almost 1 in 5 of these civilians will struggle with lingering, often subtle symptoms - headaches, dizziness, concentration difficulties and personality changes - for a year, and often longer.
As their memories falter, their work suffers and their relationships fray, many victims of brain trauma don't realize that their cognitive struggles are related to a blow to the head.
In what has been called a silent epidemic, about 2 percent of the U.S. population - 5.3 million people - cope with long-term disabilities from such accidents.
Many returning U.S. troops know those symptoms well. In almost eight years of fighting in Afghanistan and more than six in Iraq, bomb blasts, vehicle crashes and other hazards of military duty are thought to have exacted between 48,000 and 360,000 traumatic brain injuries, mostly concussions, among service members.
But their epidemic has been anything but silent. Philanthropists have endowed centers to care for uniformed victims. Lawmakers have earmarked funds to research their affliction. Communities have rallied to help with their needs. The resulting recognition has brought some comfort to civilians who struggle with wounded brains.
More important, the nation's military branches and veterans agency have invested heavily in research on brain injury and its aftermath. The combined surge of public attention and military largess, experts say, promises to bring civilian and military victims better means of diagnosing and treating brain trauma and limiting its toll.
"We're interested in finding solutions, no matter where they come from," said Dr. E. Melissa Kaime, a Navy captain who directs a program of medical research on priorities established by Congress.
The office's budget, which represents a small fraction of defense funds going toward the study of brain injury, has set aside about $300 million in the last three years for research on the subject. In 2008, the Pentagon's expenditure on brain-injury treatment and research was almost $1 billion.
The military research has prompted greater use of new medical imaging techniques that let doctors detect brain abnormalities they couldn't see before. Hand-held computer tests and back-pocket concussion-assessment cards - devised for professional sports teams but refined and promoted by the military - are making their way to the sidelines of youth sports.
And military research has prompted trauma physicians to look for spasms in the brain's blood vessels after an injury - a sign that more aggressive actions must be taken to avert death and post-traumatic epilepsy.
More improvements, including blood tests that could detect the severity of brain trauma, are on the horizon.
"Our motto is 'learn as we treat,'" said Col. Michael S. Jaffee, a physician who directs the Defense and Veterans Brain Injury Center. "That allows us to better identify those things that need to be researched. And it allows those advances that do get developed to be translated into practice more quickly."
Military physicians initially may have turned to the civilian medical community - in particular, to the neurologists and neurosurgeons who advise professional sports teams - to guide their treatment of brain injury, said Dr. Jamshid Ghajar, a Cornell University neurosurgeon who directs the Brain Trauma Foundation.
But now, the Pentagon's money, experience and team approach have turned the tables, making the military the leader in the field, he said.
"They will be, and they are, ahead of the civilian sector, and there's going to be a rapid transfer of knowledge," Ghajar said.
The principal focus of the military's research has been on so-called mild traumatic brain injury, or concussion - estimated to account for 4 in 5 of the brain injuries that affect veterans returning from Iraq and Afghanistan.
And although veterans may have additional challenges, including amputated limbs and post traumatic stress disorder, their brain injuries have much in common with those sustained by civilian victims.
To improve the prevention, diagnosis and treatment of concussion, the Pentagon and Veterans Affairs have recruited widely.
They garnered specialists who have never worked together and created collaborations among researchers who had never spoken to one another.
They enlisted sports-medicine physicians. They have coaxed neuroscientists to glean insights about brain trauma from their studies of stroke, degenerative brain disease and mental illness.
They tapped the talents of engineers, biochemists, helmet designers and gadget freaks to invent new ways of measuring blows to the head, protecting the brain from damage, diagnosing traumatic brain injury and restoring the brain's function more quickly.
The advances being made are the result of increased funding and researchers flocking to the field, said Dr. James Kelly, a University of Colorado neurosurgeon who last spring was tapped to direct a newly built Defense Department center for the study and treatment of psychological health and traumatic brain injury in service members.
"It's always been an uphill struggle for those of us who do this," Kelly said.The military turned to sports-medicine specialists for help making a decision that coaches and players face when they suspect a collision has resulted in concussion: whether to take a player out of the game or a soldier off the battlefield.
The result of the collaboration was the MACE card - short for Military Acute Concussion Evaluation - that every military medic in Iraq or Afghanistan carries. It is a paper-and-pencil adaptation of a computer-based test called ImPACT, designed for coaches and trainers to gauge the presence and severity of concussive symptoms.
Such common ground includes a growing concern for the long-term effects of these injuries.
In September 2008, the Defense Department and the NFL convened a conference to share news and research on traumatic brain injury, including the biomechanics of brain injury, new helmet designs, concussion prevention, and better ways to diagnose and assess traumatic brain injury on the sidelines and in the hospital.
"Warfare is a curse on humanity, but the things that are learned in warfare invariably translate to civilian use," said Dr. Kenneth C. Curley, the Army's neurotrauma research coordinator.
And the population of aging professional athletes - unstudied until recently - may provide military veterans and those who care for them a revealing glimpse of their future.
Already, studies of retired NFL players have yielded links between recurrent concussions and elevated rates of depression, mild cognitive impairment and earlier onset of Alzheimer's disease.
