Versus mode remained as is, while Campaign mode invariably has players as the Survivors versus AI-controlled Infected. Valve had difficulty with keeping the game balanced for both teams and they eventually settled on splitting the game into two modes.
Left 4 Dead was originally going to just have the Versus mode, where the Survivor team had to reach the safe house without being killed by the infected players (or get closer to it than the other team) and then the teams switched at the end of the round.While it was scrapped, Valve thought some sort of co-op game would be better instead which eventually led to Left 4 Dead. Before Left 4 Dead started development, Valve worked on a fantasy game involving fairies.
