Contagion

Source Engine-based Contagion is another attempt at zombie apocalypse theme in the FPS genre. This attempt was quite successful at the launch, and recently developers came back to regular updates. The game consists of a singleplayer campaign and multiplayer mode. It's recommended to use singleplayer as training before playing with others online. Apart from that, each round is randomly generated in terms of zombie presence and level of difficulty. This is made on purpose for every play through to be unique. Speaking of multiplayer, you're offered to choose from a wide variety of modes. Some of them require you to apply survival skills to rescue civilians and escape the hordes of living dead. The classic Deathmatch includes clashes between players across several vast maps. Contagion is fully moddable and contains a level editor for you to design your own game experience and play it through with your friends.