Valve has said that Left 4 Dead 2 add-on The Passing is due out in March.
The news came from Microsoft's X10 event in San Francisco last night where Valve's Chet Faliszek also told Kotaku that Left 4 Dead 1 would get an add-on about a month later.
That chunk will tell the story of how the original survivors ended up getting to where they are when you encounter them in The Passing.
According to Faliszek, The Passing itself will see the two groups - you as the L4D2 cast and the originals as NPCs - joining forces to deal with one of its chapters.
There's also a new uncommon common infected, the Fallen Survivor, who occasionally drops items like health kits when dispatched.
Pricing for The Passing will be announced closer to the launch.
Elsewhere, Valve has revealed that gamers dispatched 28,981,249,043 zombies in the first two months of L4D2's existence, and produced some funky stats to contextualise the figure, like how tall the zombie tower would be if you stacked the bodies head to toe.