Left 4 Dead popularized the co-op action zombie game genre for sure, but the series certainly isn't the only game in town.  Killing Floor started as a Half-Life mod and moved into full blown territory shortly after and Contagion looks to do the same having come from the creator of the Half-Life 2 mod called Zombie Panic: Source.   We went hands on with the game at PAX Prime 2012 and came away impressed with some truly unique gameplay elements.