Do we Think PTSD is real and why

Ok I'm going out on a Limb here My Personnel Belief is it's a Crock. I served as a Member of a FO Team in a Line Inf Unit in Nam. I have walked through Basecamps after call Fire for hours on it seeing nothing but Death and Destruction,Called for Fire almost on top of myself, saw my Friends brought off Battle Field in Cargo nets under a Helicopter Brought my Best friend Home to his Mother after He and My replacement were Killed 12 hrs.' after I Left the Bush. So yes I saw and Lived through it all. But other then an Occasional Nightmare, it hasn't effected my Life what so ever. Now Granted it could be that I elected to remain in the Army for the next 23 years and had the support of my Brothers in Arms. One more thing that causes me Dubt is that when I check on the Background of the who's who are talking the Loudest most have the same thing in common, Non Combat Arms. I would Like to recommend a Book called Stolen Valor, check out the part about PTSD.
So now I have said my Peace Let me Have it.