Virtual Iraq.

In November, 2004, when he was nineteen years old, a marine I'll call Travis Boyd found himself about to rush the roof of the tallest building in the northern end of Falluja in the midst of a firefight. Boyd, whose first assignment in Iraq was to the security detail at Abu Ghraib prison, had been patrolling the city with his thirteen-man infantry squad, rooting out insurgents and sleeping on the floors of abandoned houses, where they'd often have to remove dead bodies in order to lay out their bedrolls.

With Boyd in the lead, the marines ran up the building's four flights of stairs. When they reached the top, "the enemy cut loose at us with everything they had," he recalled. "Bullets were exploding like firecrackers all around us." Boyd paused and his team leader, whom he thought of as an older brother, ran past him to the far side of the building. Moments after he got there, he was shot dead. Within minutes, everyone else on the roof was wounded. "We had to crawl out of there," said Boyd, who was hit with shrapnel and suffered a concussion, earning a Purple Heart. "That was my worst day."

It is in the nature of soldiers to put emotions aside, and that is what Boyd did for three years. He "stayed on the line" with his squad and finished his tour of duty the following June, married his high-school girlfriend, and soon afterward began training for his second Iraq deployment, not thinking much about what he had seen or done during the first. Haditha, where he was sent in the fall of 2005, was calmer than Falluja. There were roadside bombs, but no direct attacks. Boyd was now a team leader, and he and his men patrolled the streets like police. When drivers did not respond to the soldiers' efforts to get them to stop, he said, "we'd have to light them up." He was there for seven months.

With one more year of service left on his commitment, and not enough time for a third deployment, Boyd was separated from his unit and assigned to fold towels and clean equipment at the fitness center of his Stateside base. It was a quiet, undemanding job, intended to allow him to decompress from combat. Instead, he was haunted by memories of Iraq. He couldn't sleep. His mind raced. He was edgy, guilt-racked, depressed. He could barely do his job.

"I'd avoid crowds, I'd avoid driving, I'd avoid going out at night," he told me the first time we spoke. "I'd avoid people who weren't infantry, the ones who hadn't been bleeding and dying and going weeks and months without showers and eating M.R.E.s. I'd have my wife drive me if I had to go off the base. A few times, I thought I saw a mortar in the road and reached for the steering wheel. I was always on alert, ready for anything to happen at any time."

Eventually, as part of a standard medical screening, Boyd was diagnosed as having chronic post-traumatic stress disorder. P.T.S.D., which in earlier conflicts was known as battle fatigue or shell shock but is not exclusively war-related, has been an officially recognized medical condition since 1980, when it entered the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders. (In an earlier edition, it was called "gross stress reaction.") P.T.S.D. is precipitated by a terrifying event or situation--war, a car accident, rape, planes crashing into the World Trade Center--and is characterized by nightmares, flashbacks, and intrusive and uncontrollable thoughts, as well as by emotional detachment, numbness, jumpiness, anger, and avoidance. Boyd's doctor prescribed medicine for his insomnia and encouraged him to seek out psychotherapy, telling him about an experimental treatment option called Virtual Iraq, in which patients worked through their combat trauma in a computer-simulated environment. The portal was a head-mounted display (a helmet with a pair of video goggles), earphones, a scent-producing machine, and a modified version of Full Spectrum Warrior, a popular video game.

When Travis Boyd agreed to become a subject in the Virtual Iraq clinical trial, in the spring of 2007, he became one of about thirty-five active-duty and former members of the military to use the program to treat their psychological wounds. Currently, the Department of Defense is testing Virtual Iraq--one of three virtual-reality programs it has funded for P.T.S.D. treatment, and the only one aimed at "ground pounders" like Boyd--in six locations, including the Naval Medical Center San Diego, Walter Reed Army Medical Center, in Washington, D.C., and Weill Cornell Medical College, in New York. According to a recent study by the RAND Corporation, nearly twenty per cent of Iraq and Afghanistan war veterans are suffering from P.T.S.D. or major depression. Almost half ...