Who would have thunk it? We’re well into the zombie craze, even though some among us think we should be well beyond it, and another post-apocalyptic title goes all “smash hit” on us overnight. This time it’s the currently Xbox LIVE exclusive State of Decay that’s earned a spot in the limelight.

Xbox Wire, the official news site for Xbox goodness, has announced that State of Decay has officially become the fastest selling original Xbox LIVE Arcade game ever. Since its release on June 5th, it has over 550,000 downloads.

Xbox Wire asked someone from Undead Labs, the studio behind State of Decay about the success and whether or not it was a surprise. Here’s their genuine response:

We didn’t know what to expect. “State of Decay” breaks a lot of rules. Permadeath, switching characters, having to manage the “feelings” of your fellow survivors, offline progression — these are all things developers traditionally avoid, but we felt were essential to creating a true survival experience. We’re thrilled and humbled that gamers are embracing the game. Record breaking sales are great, but the thing we’re proudest of is that people who bought the game are having a great time playing it. That’s why we do what we do, and what keeps us going back in to do it again.

State of Decay is available on the Xbox LIVE Arcade right now for 1,600 moon dollars (that would be $20 in real money). There’s also a free demo for you to sample. The game will eventually hit the PC platform by way of Steam, though that release date has not been announced.

We’ll have a “We Play” of this title soon. Stay tuned!