In Vietnam, "Roy" killed too many people to remember a final tally.

Some of the kills came during hand-to-hand combat, of which Roy, 63, has exceedingly vivid memories.

The horrors of combat were a long way from the pastoral life Roy knew on a cattle ranch in Idaho as a child or the safety of college classrooms as a young man.

The war would change him in ways he couldn't have imagined.

Roy, who requested that his real name not be used, was a Marine corporal in Vietnam. He earned two Purple Heart awards after his time on the front lines. He killed to live another day.

"I never felt guilty about what I did over there," Roy said recently in an interview.

But by the time he received an honorable discharge in 1970, Roy knew his heart had already begun turning cold and black.

Back then there wasn't the kind of help that's available through today's transition assistance coordinator, a person who helps veterans readjust to civilian life after returning from a tour in Iraq or Afghanistan.

Roy, who lives in Sandy and wished to remain anonymous for this story, was expected to adjust on his own after the war. It was a time, he said, when the government, society and his own family turned their backs on him.

What might have been called "battle fatigue" back then began changing who Roy was. He was — and still is — angry and to a degree antisocial. He was violent, having beat a police officer senseless back in the '70s. He still prefers to avoid crowded, cramped places with too many strangers and no quick exit.

What changed Roy forever is now called post-traumatic stress disorder (PTSD), something that affected about half of all Vietnam vets and impacts about one-third of military members coming back from Iraq and Afghanistan.

Defining PTSD

It's estimated that one in two people will be exposed to some kind of traumatic event in their lives that could lead to PTSD. Among these events are military combat, personal assault, natural disaster, a traffic accident or being diagnosed with a "frightening" illness, according to the Web site ptsd.factsforhealth.org.

About 4 percent of Americans have PTSD at any given time. It's not clear why some trauma victims develop PTSD and some don't. Statistically, women appear to be twice as likely as men to be diagnosed with PTSD.

There is more likelihood of developing PTSD after very severe trauma and if the event was something that occurred between people (as opposed to a natural disaster or learning about an illness.) Those who are more vulnerable to PTSD include children, the elderly, people with "poor social support" and those with an existing disorder linked to anxiety, depression or personality.