No such thing as a Left 4 Dead prequel, it was all a rumour13 Jun 2012
The story was that Overkill Entertainment (Payday: The Heist) were working on an unannounced Left 4 Dead game that would tell the story of how the zombie outbreak came to be. Valve have since snubbed the rumour:
“We’re trying to be a little coy and a little fun because we want the communities to discover it,” Valve’ Chet Falisek told PCGamesN, “but those guys are still making Payday: The Heist, we’re still making Left 4 Dead,” said Valve’s Chet Falisek.
Chet said that there is a collabration going on between Payday and Left 4 Dead but that it “is kind of akin to how in TF2 you might see some other games’ weapons get translated and put into that game. We’re doing something similar to that, but it’s a little bit more involved.”