The DLC will include a new multiplayer mode titled Survival, as well as two complete campaigns for the game's existing Versus Mode. No details regarding availability or pricing were announced, but Valve traditionally updates its PC releases for free.
Valve did not specify exactly what new content the Survival Mode will pack, aside from the new mode. The studio previously said that it was working on new scenarios.
A Critic's Choice Edition of the game will head to retail stores at the same time, offering the original game plus the Survival Pack content.
Valve also noted it will release the Software Development Kit for the game this spring. With the SDK, players will be able to create custom campaigns and levels for the game.
Distributor Electronic Arts recently revealed that Left 4 Dead has sold 1.8 million copies at retailers since the game's November 18 release. That number does not include Steam, as Valve does not release those figures publicly, or total worldwide sales.