Valve has announced that DLC for Left 4 Dead 2 is on the way. "The Passing" will be available in March, and will bridge the gap between L4D and L4D 2. Somehow Left 4 Dead 2's gang will meet up with the cast of Left 4 Dead, although the survivors from the first game will be NPCs.
"The Passing" will also add a new infected type, The Fallen Survivor. These are humans who believed themselves to be immune to the infection, but ended up zombified anyway. They will sometimes drop health kits or ammo when killed. It will include new arenas for Survival, Versus and Scavenge modes, and feature Hot, a new co-op challenge mode.
If you haven't purchased Left 4 Dead 2, don't worry. Valve doesn't abandon the community that still plays their older games. Case-in-Point: Valve is going to release downloadable content for Left 4 Dead to complement the release of "The Passing" DLC for Left 4 Dead 2. We don't know much about the Left 4 Dead DLC yet, other than that it tells the story of how the survivors from the game got to that point, but stay tuned; we'll find out eventually.
Finally, if you don't pay for either Left 4 Dead game, you can enjoy a comic book based on the games. "We actually have a comic book coming out between the two DLCs, which will help us give a little bit more of a back story to explain what's happening in the world," Valve's Chet Faliszek told Computer and Video Games.