The next update to Left 4 Dead 2, called The Passing, is up for release in March Valve revealed at X10. In the expansion, the new survivors will meet up with the characters from the first game to bring fiery death to the zombies including a new special infected; Fallen Survivor. Valve's Chet Faliszek also revealed to Kotaku that the first game will see an update a month later, where the story tying the two games together will be told.
In their own special way Valve have also revealed that in the two months following Left 4 Dead 2's release, players have killed an impressive 28,981,249,043 zombies.