Zombie Panic: Source is a free Co - Operative game mod downloadable of Steam. This game mod is more along the lines of a whole other game. It lets you play as either a zombie or a survivor. Because you can play as a zombie, it gives the player a new perspective, allowing you to make more intuitive descisions as a survivor. Mission types vary from survival to escape to antivirus retrival. A previously available mission even had you run to a bunker and set off a nuke, eleminating the remaining zombies. however, this mission is now unavailable.