Despite denying a crossover at E3, Valve is indeed working with PAYDAY: The Heist developer Overkill Software on a heist level called PAYDAY: No Mercy, set in Mercy Hospital. From the trailer, it appears the bank robbers from Payday begin the infection that leads to the events of Left 4 Dead.
1. 0:34-0:37: “We’re here to draw some blood from a guy that they have in quarantine.”
2. 1:39: “You’re [beep] 4 dead.”
I can’t wait to test this level out and explore some L4D lore; I also really like the fact that Valve is letting other devs experiment with their franchises. Let’s hope a release date is announced soon.