The figures quoted vary widely round the world - here is the latest estimate from Australia as reported by ABC News:
A mental health expert believes up to 10 per cent of war veterans are suffering from post-traumatic stress disorder (PTSD) and the condition is leading to cardiovascular disease.
Associate Professor Mal Hopwood from Austin Health in Melbourne has addressed the Congress of the Royal Australian and New Zealand College of Psychiatrists in Adelaide.
Professor Hopwood says the Australian Defence Force (ADF) has made improvements in treating PTSD, but more needs to be done.
"One of the current areas of concern is the gap some people experience between leaving the ADF and then receiving mental health care as an entitled veteran," he said.
"Sometimes that gap can take several years and clearly that's not ideal in terms of getting early intervention for mental health problems."