l4d2Valve’s zombie-killing Left 4 Dead 2 video game has sold more than 2 million units on the Xbox 360 and the PC since its debut two weeks ago. That’s important because it shows that the holiday season isn’t just about Modern Warfare 2 (or about killing just the living).

Bellevue, Wash.-based Valve said the zombie apocalypse sequel doubled sales of the original Left 4 Dead launched a year ago. The company also said that Black Friday promotions helped boost sales and that more than a million members of Xbox Live have signed on to play the game.

It’s not as big a hit as Modern Warfare 2, which generated $550 million in sales in its first five days. But it shows that, despite the Modern Warfare 2 blast radius, there is still room for big hits beyond Ubisoft’s Assassin’s Creed II, which launched last week. The video game industry needs a strong November and December to make up for the rest of 2009, which has seen a decline in sales compared to a year ago.

It’s also good news for Electronic Arts, whose EA Partners division published the game developed by Valve. EA Partners is a small group with a few dozen people, but it has generated hundreds of millions of dollars for Electronic Arts via distribution deals with high-profile developers.

In the game, you play one of four people in a disaster-hit New Orleans who grab weapons and fend off hordes of zombie attackers, who move extremely fast and surround you within seconds. Your strategy is to fire as much as possible and make sure you don’t run out of ammo. You also have to spend a lot of time making sure your computer-controlled or multiplayer human partners aren’t overwhelmed by zombies. It’s a bloody mess, but more thrilling than typical zombie-shooting games where the zombies move at slow speeds. Your weapons include chainsaws, frying pans, axes, baseball bats and a variety of guns. I’ve played some of the game. The initial level is cool — you have to deal with fighting zombies and escaping the flames of a burning hotel at the same time. I plan to play more after I’ve had my fill of the multiplayer version of Modern Warfare 2.

The game uses procedurally-generated effects, meaning they are calculated and created on the fly, rather than prefabricated. That means that objects in the game, pathways, and weather effects change from scene to scene. The number of enemies also depends on how well you play. As a result, the game is infinitely replayable. There’s also more of a story to this version with new kinds of boss zombies.