Marine Lance Cpl. Gene Landrus was hurt in a roadside bomb attack outside Abu Ghraib, Iraq, on May 15, 2006, and faces medical separation from the Corps. He's also up for a Purple Heart.

Along with 20,000 other veterans, he's not included in the Pentagon's official count of U.S. troops wounded in Iraq and Afghanistan.

That's because Landrus' wound was to his brain and hidden from view. Landrus, 24, of Clarkston, Wash., says he did not realize the nausea, dizziness, memory loss and headaches he suffered after the blast were signs of a lasting brain injury.

Army medics who examined him in the field didn't find the wound, either. "They wanted to know if we had any holes in us, or if we were bleeding. We were in and out of there (the aid station) in 10 to 15 minutes," Landrus remembers.

For the balance of his combat tour, he tried to shake off the blast's effects and keep going. Now, "my goal is to get back to a normal life," he says.

A USA Today survey of four military installations and the Department of Veterans Affairs, where combat veterans are routinely screened for brain injury, has found that about 20,000 people show signs of damage. They are not counted in the Pentagon's official tally of 30,000 war wounded.

The military lacks "a standardized definition of traumatic injury or a uniform process to report all TBI (traumatic brain injury) cases," Assistant Secretary of Defense Ellen Embrey wrote in a memo last month. As a result, it is hard to determine the scope of the problem, she wrote.

The military hopes to address both issues soon, says Army Col. Robert Labutta, a neurologist and brain injury consultant to the Pentagon.

Military medics are now trained to uncover signs of brain injury on the battlefield, says Air Force Lt. Col. Mike Jaffee, interim head of the Defense and Veterans Brain Injury Center, which is devoted to treatment and research. Brain-injury screening questions have been added to medical surveys given to all troops returning from war.

Landrus was riding in an open-backed, armored Humvee when the roadside bomb detonated. It was his second exposure to a blast. An explosion a month before had "rung our bells a little bit, but no one was knocked unconscious."

In the attack May 15, 2006, Landrus and three other Marines blacked out for several seconds. After Landrus regained consciousness, "everything looked like it was going in slow motion," he recalls.

The battalion came home in August 2006 to Camp Pendleton, Calif., one of a few military installations that screens for brain injuries among returning troops. Landrus was referred to Navy doctors who diagnosed brain injury. With medication and rehabilitation training at nearby Scripps Memorial Hospital Encinitas, Landrus has improved.

"I still can't remember what I did the day before or stuff that I did earlier in the day," he says. He carries a Palm Pilot or a pad of paper to write down orders, numbers or dates, so he can remember them later. The headaches have never gone away.

Landrus will never fully recover, says Jessica Martinez, his lead therapist at Scripps.

"This is basically like an invisible injury," she says. "He looks like a normal guy. ... But if you spend any amount of time with him ... you would be able to notice that something's really happened."