What is the story behind the left 4 dead games

A town is infected with a virus (first game, much like most zombie games, dunno about second) and your team of "4" survivors are placed randomly throughout the mock town and have to find a way out. It is also a mock movie, at the end it gives credits saying x amount of zombies were killed in the making of this film. Also, the achievement for xbox 369, kill 50 some odd thousand zombies is a reference to the population of the town. The next few zombie games "one up" each other making it to where you have to kill one more zombie than in the last game to get the achievement.