Symptoms usually start soon after a traumatic event, but may not emerge until months or years later, according to the National Center for PTSD, run by the Department of Veterans Affairs.
Sufferers can relive the event in nightmares and flashbacks or even when just hearing a car back fire or seeing a car accident. Emotional numbness, hyperarousal and feelings of hopelessness are also symptoms.
Rates of post-traumatic stress disorder among troops serving nearly a decade in Afghanistan and Iraq have been on the rise and has been directly related directly to combat exposure. Soldiers at greatest risk were under the age of 25, according to 2009 ABC reports. Suicides in that age group were also up.
In May, the American Psychiatric Association will devote part of it upcoming annual meeting to promising approaches in intervention and treatment in the military.
"It's a hot topic -- we are a nation at war," said Marmar, who will run one of the workshops on advances in neuroscience with implications for treating and preventing PTSD.
Those most vulnerable have a family history of mental disorders or have been exposed to traumatic stressors in childhood. They are more likely to have a lower educational level, and not have had extensive training for their military role or cohesion, according to Marmar.
Higher rates of PTSD are also observed in younger, active-duty soldiers, perhaps because they are "the tip of the sword and see more explosions," he said. Older National Guard and reserves are also more prone because their lives have been disrupted.
With each deployment the chances of getting PTSD increase. "Three tours is worse than two tours, and two is worse than one," he said.
Under the leadership of Comdr. Charles Benson, psychiatrist in charge in Helmand Province, the Marine Corps and Navy have launched the Operational Stress Control and Readiness Program
Psychiatrists and psychologists are embedded in the regiments and battalions, living with troops and out in the field with them.
"It kind of breaks down the barriers and allows them to become very effective in their jobs delivering mental health care," said Benson said at a press conference in January.
The mental health team identifies and helps Marines with "simple issues" and determines when they are to be referred for higher care, he said.
"We'll see what the outcomes are," said Marmar, who noted that NYU studies on Vietnam War veterans showed that those who did well right after coming home, "in the long term weren't good."
"We won't know for the next five to 10 years," he said. "The jury is still out. Right now is the honeymoon effect, because they are coming home. They are so grateful to be alive."
PTSD can take time to evolve, sometimes being triggered by another traumatic event that has nothing to do with war. And the number of veterans diagnosed in initial screenings is always lower than in follow-ups.
The returning Marines are gearing up for more mental health screenings as they reintegrate. First there will be a remembrance ceremony for those who did not come back, and then they'll have a vacation block.
By June, said Carlton, they will be back at work on the base, unlikely to be redeployed to a war zone for at least a year.
Their spirits are high, he said. "There is something to be said for accomplishing a purpose. Everyone in the 3rd Battalion, 5th Regiment is proud of what they have done. We knew what we were getting in to."
"I am being selfish here, but this is the finest group of men and women I have ever come across," said Carlton. "I thoroughly believe that Afghanistan is a better place and these guys will look back with pride in what they have accomplished in the last eight months."
Luis Martinez contributed to this report.