Left 4 Dead Syndrome 

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."

Left 4 Dead Syndrome by MOON-E December 30, 2009

Left 4 Dead Syndrome 

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

Left 4 Dead Syndrome by Apocalypso January 26, 2010