Left 4 Dead

Left 4 Dead is a cooperative shooter with up to 4 players working together to escape a zombie apocalypse. Zombies (the "infected") are controlled by sophisticated AI that takes realistic paths and can scale any distance. The "Director" AI changes the intensity and difficulty of the game with the players' performance, so each game is unique.

Left 4 Dead was built off Orange Box and Counter-Strike: Source code.