The devs are focused on "Contagion," the follow up that is more like L4D than ZPS. And they are focused on it cause they want you to pay them $$$ for it...
Problem they are going to face is, their game's name corresponds with a movie coming out this year, about a contagious virus which causes zombies... See the problem? Guess who has the $ to kill anything similar to the movie, so the movie can have a game attached? Not the ZPS devs...