Often caused by playing excessive amounts of Left 4 Dead games or any other zombie games. It is where you get to the point where you imagine what every place you go to would look like, if it were a zombie apocalypse.
Affected: Dude, imagine fighting off a zombie apocalypse in this mall! Aww man, you could even use that display car to escape.
Normal person: Wow, dude you've been playing way to much left 4 dead
Affected: Yeh, I think I've got left 4 dead syndrome