So what's the appropriate greeting for when you pass another survivor in the zombie apocalypse? We'll find out next month, when Valve plans to release the first Left 4 Dead 2 DLC campaign, "The Passing," on Xbox 360 and PC. The Passing will feature non-playable appearances by the original Left 4 Dead cast (those shadowy figures in the background there), who will aid L4D2's four survivors during the third act of the new campaign, which is set in rural Georgia.

Not one to leave holes in a good story, Valve will explain the original survivors' frantic journey to Georgia by way of -- yes, it's really happening -- a new DLC add-on for Left 4 Dead 1 (not compatible with L4D2). No additional details were provided about the upcoming L4D1 DLC by Valve at X10 today, but it will follow the release of The Passing campaign and a digital comic (illustrated by Michael Avon Oeming) that will tie together the two DLC stories.

Additionally, Valve confirmed that The Passing will also include new arenas for Survival, Versus and Scavenge modes, and a new co-op challenge mode. Players will also encounter a new "uncommon common" infected, dubbed the Fallen Survivor, which randomly drop items when killed. The Passing is currently on track for a "late March" release.