Valve's Chet Faliszek told Kotaku that it will not reveal the origins of the infection and is not part of the Left 4 Dead canon.
"This is a 100% Overkill-made mission for Payday: The Heist simply set in No Mercy hospital from the Left 4 Dead series," insisted Faliszek.
"It is not telling the origins of the Infection and is not canon. There is a little cameo from one of the L4D characters 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."
Announced last month, Payday: The Heist - No Mercy is described as an in-depth collaboration between the two gaming franchises.
"We are working on a very cool blend of Payday and Left 4 Dead. I am sure it is so exciting that it will have some players check into the hospital before we are done," revealed Overkill's Ulf Andersson.
Payday: The Heist is a first-person shooter that lets players take part in bank robberies, prison breaks and drug ring infiltrations in teams of four.
It can be downloaded for the PlayStation 3 and PC.
Making its debut in 2008, Left 4 Dead sees teams of four strangers attempt to survive a zombie outbreak.
A sequel was released in 2009, followed by a number of additional levels and gameplay enhancements.
> Read Digital Spy's hands-on preview of Payday: The Heist
> Read Digital Spy's Left 4 Dead 2 review
Watch a trailer for Payday: The Heist below: