It can be both. You can have one fairly significant traumatic event, or it can be an accumulation of a number of smaller events.
In my experience, the soldiers, the men and women who develop PTSD in theatre—it doesn't have to be in theatre; it can be in Canada—are able to continue doing their jobs. Their problem is when they come home. Particularly, they have problems when they have to interact with their family and friends and stuff. That's where it really shows up.
I've sent people back on tours with PTSD and they've done okay. The problem is when they get back home.