Zombie Panic! Source is a standalone cooperative, survival-horror, first-person-shooter! Set in the middle of a zombie apocalypse, players start as a small group of survivors attempting to stay alive. Each map will have its own objectives to complete and win the round or survive by a period of time.
This game, simply put, is amazing. Anyone that ever enjoyed a zombie game, an infection esque type game mode, a survival horror game, or just video games in general needs to play this game.