I have PTSD, from military service in Iraq. My PTSD is not me but it is my burden. Like Becvan stated before me, it is a survival skill. Here is something I would like to share more of about me in regards to this.
Everytime I go someplace I am watching everyone, observing and judging people. I am observing there behavior for signs of danger and judging on wiether that person is a threat. Through my entire stay I watch everyone, I have my back in a corner so no one can be behind me, if I am not the sensation of danger and fear is compounded greatly by my thought of vunerablility. Where is my backup, where are my fellow soliders. I may begin to shake, just my leg or legs at first, then my hands. My sences sharpen as I anticipate an attack, I go into what I call my combat awareness stance. I get the "100 yard" look to me as I mentally catalog everyone's actions in the room. This is less of an issue the more I visit a place, I have been going to the same bar near my home for 2 years, I still get scared but atleast I dont have to sit in the corner anymore.
This is not an illness, this is me surviving. This is a result of me surviving in a full year of wondering if I was going to get hit by a mortar while I walked to a sand burm to take a crap, every aspect of your day/night truely revolved around the chance of being hit without warning. A result of driving down the narrow roads of Tikrit surrounded by thousands of civilians, many convoys, many chances with death.