CVG has it that Payday: The Heist developer Overkill is working on a Left 4 Dead collaboration with Valve.
It is thought the prequel will explain how the outbreak actually happened to begin with it.
Last month, the studio teased something relating to Valve and L4D. It looks like this could be the prequel CVG are on about.
L4D and it’s sequel were originally released for Xbox 360 and PC.