PTSD is one of those fad diagnoses which won many adherents in the past ten years, even gaining admission to the DSM.
As I have written before, very few of the descriptive Psychiatric diagnoses have validity - all most of them (with a handful of notable exceptions) have is varying degrees of reliability. In my field, a diagnosis does not mean a disease in the usual medical sense (which is why we call them "disorders").
What is termed PTSD is presumed to be a collection of complaints which some (but not most) people experience following significant emotional trauma. There is no doubt that people are distressed by, and, I think, permanently altered by significant emotional trauma. It doesn't have to be bad experiences in combat, because many things in life can constitute emotional trauma (depending on the person's psychological make-up). The reason PTSD is so often studied in combat vets is because that's where the research money is. (In the past, such symptoms were classed as ""nervous in the service," "combat fatigue," "shell shock," "traumatic neurosis," and the like.)
Here, an anthropologist looks at post-traumatic stress.