Left 4 Dead Syndrome (or Left 4 Deads / L4DS) is a common occurrence caused by extensive amounts of Valve's video game L4D (or it's sequel L4D2). It causes the victim to be constantly paranoid, often hearing noises such as various special infected or the Tank! music whilst outside of the game. Extreme cases may also cause the victim to down entire bottles of pain medication or yell out their actions such as "RELOADING!"
"Bob was relieved to discover the smoker he thought he had heard was just a normal man coughing, and that he was simply experiencing Left 4 Dead Syndrome"
"Dude, I think I have Left 4 Deads, I could have sworn I heard a tank back there."
awesome with a k, originating from the lack of k-adjectives use to describe people who's names start with K when participating in an exercise involving the participant describing themselves with an adjective starting with the same letter as their name