When Left 4 Dead first introduced its new brand of co-operative zombie-slaying, it didn't take long for a rabid fan-base to develop. In a recent interview, several Left 4 Dead luminaries, including writer Chet Faliszek and then-Turtle Rock CEO Mike Booth, reminisced about how the Left 4 Dead series came to be.

The team revealed that Left 4 Dead's DNA actually came from a Counter-Strike mod. Shortly after Turtle Rock Studios had shipped Counter-Strike: Condition Zero in 2004, CEO Mike Booth showed a mod called Terror Strike--ostensibly a never-ending zombie-attack mode on the CS_Italy map--to Faliszek and Erik Wolpaw, who both fell in-love with the concept. Later that day, Valve CEO Gabe Newell conscripted Faliszek and Wolpaw to team up with Booth's Turtle Rock (purchased by Valve in 2008) to develop the concept further. Early in development, Booth explained that the team understood they "had this nugget of gameplay where a small co-operative group had to deal with hundreds of melee monsters."

Years of playtesting and iteration helped the team focus on trying to create an experience for players that was both "emergent and yet structured." Out of this focus came the game's procedural population system, which constantly adds and removes zombies from the game world to create the illusion of never-ending hordes of the undead. (The first game only allowed 30 on-screen zombies at a time.) According to Booth, the zombies' non-aggressive ambient behavior (such as when they're just standing or laying around) was also based on fears brought about by the potential 2005 bird-flu pandemic.

Booth clarified:

We kind of pushed on that with the wandering infected, how they stumble around and vomit and just look like they're having the worst flu ever. We wanted that combination of pity and 'it could be me’ with 'this is horrible' and then 'Oh my God, here they come, we have to survive.'

The game's now-famous AI-director--which modulates when and where zombies and item pickups will appear, based on a real-time assessment of player performance--was also created in aid of a more dynamic, less predictable experience. Booth explained how Left 4 Dead's AI-director evolved along with the series:

We needed to make sure that certain tempos and pacing happened on a regular basis to keep people's excitement and attention going. For L4D that was basically just me and some C++ code making that happen. In L4D2 we generalised it into a larger tools framework.

The Left 4 Dead retrospective, which also touches on competitive multiplayer and how the team designed the game to encourage players to cooperate in a world where co-op was still a relative rarity, can be found here.