In March, Valve will release its first DLC for its hit zombie shooter sequel Left 4 Dead 2. Taking place after the "Dead Center" campaign and before the "Dark Carnival" campaign, "The Passing" will feature not only the usual gang from Left 4 Dead 2, but also the original survivors from Left 4 Dead 1.
Valve is planning on releasing another piece of DLC for Left 4 Dead 1 to arrive a month after "The Passing." Supposedly, the new campaign will explain how the original set of survivors ended up meeting the newer survivors. That's a lot of chronological tweaking that should make fans very happy. The second piece of DLC would be only for Left 4 Dead 1.
For now, here are some screenshots and the poster for "The Passing."

Who's that lurking in the shadows?

Probably should of stayed at Club D.

Coach is either dead already or buying a snack at Red Flight.

Might want to go back in the safe room and get more than just a pistol.
For more screenshots, head here. "The Passing" comes out at the end of March.
Source: IGN