Despite some vague-ish comments from Valve that this wasn't the case, IGN has a video from the Payday: The Heist YouTube channel indicating that developers Overkill are working with Valve on a game called No Mercy that will explore the origins of the zombie virus from Left 4 Dead.
The video at IGN depicts players fighting through Mercy Hospital, the setting for a famous level in the original Left 4 Dead. The level looks like a heist gone wrong (or at least violent), and before long things have gotten hairy. Left 4 Dead flashlight-shooting ensues, and I gotta say it's weird to see the things at the other end of the flashlight beams shooting back.
Update: Valve's Chet Faliszek has emailed us, saying that the YouTube video isn't a prequel, that it was "just some over-excited marketing guys."
"This is a 100% Overkill made mission for Payday: Heist simply set in no mercy hospital from the Left 4 Dead series. It is not telling the origins of the Infection and is not canon. There is a little cameo from one of the L4D characters (the comic should give you a hint) and a fun little easter egg with a payoff later in Left 4 Dead. Just us having some fun with a studio we are friends with and who have also made a great co-op game."
Before it was pulled, the YouTube description said, "Have you ever wondered how the Left 4 Dead series began? It started with a heist!" The level looks like a heist gone wrong (or at least, gone violent), and before long things have gotten hairy. Left 4 Dead flashlight-shooting ensues, and I gotta say it's weird to see the things at the other end of the flashlight beams shooting back.