Stanley MacDonald is a member of the Third Battalion of Princess Patricia's Canadian Light Infantry. During a routine drug interdiction operation, MacDonald and his team come across the aftermath of a zombie attack.
Okay, so you know how every horror story has the guy who sees the horror first, and then no one believes him? In World War Z, that guy is none other than Stanley MacDonald. Yeah, if there's one guy you don't want to be, it's that guy.
MacDonald attempts to tell his superiors about his zombie encounter, but they diagnose him with PTSD and send on his merry way. This disregard of MacDonald's story demonstrates one of many early examples of government ineptitude. (Don't worry if you missed it in this story as there will be plenty more examples to come.)
Worse still, MacDonald admits, "They didn't break me, I broke myself[;] I wanted to believe them and I wanted them to help me" (2.3.13). This statement links him to other characters like Mrs. Miller, whose story is about personal responsibility in the face of an irresponsible society.
We never hear from MacDonald again, but Johnny Clegg does say that MacDonald gets "the monkey off [his] back," suggesting he's coming to grips with his experiences during the war (9.3.10).