Dr Jonathan Shay, a US psychiatrist who has worked with Vietnam veterans for many years, says combat stress is an age-old problem - certainly one known to the ancient Greeks.

In his book Odysseus in America, he argues that the Homeric hero was a severe combat stress case - a loner and deceiver who had murderous rages.

"Combat stress is as old as the human species," he says - and, in a way, a very normal phenomenon.

"It is an absolutely valid adaptation to survive in a horrific situation. In war, people really are trying to kill you. You are surrounded by enemies and have to be prepared to kill instantly to survive."

Soldiers - and civilians caught up in war - become hyper-vigilant, unnaturally alert and focused.

And combat can have a devastating effect on a person's emotional health.

"We shut down all emotions that do not serve survival - grief, sweetness, fear," Dr Shay says.

But one emotion may remain switched on, he adds: anger.

"So a veteran comes home with all emotions shut down except for anger. Guess what this does in the family, in the workplace. It's a problem," he says.

'Limiting life'

Some cope with it by withdrawing from society in one way or another.

"There are numerous examples where a veteran will severely limit his life, isolating himself to protect us," Dr Shay says.

"They will tell you: 'If I go out in public, I know I'm going to meet some jerk who's going to cross me and I'll do something and spend the rest of my life in jail.' Many truly don't want to hurt other people."

Those are the extreme cases, of course. Only a minority of soldiers - even those who see combat - experience PTSD.