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 can drop random items when killed. The Passing is currently on track for a "late March" release.