Valve games typically contain stories that are expressed through exploring the world and interacting with its inhabitants rather than from watching cut-scenes or reading explanations out of the game manuals. This seems to be the case in Left 4 Dead as well. Graffiti covers many walls of the game, often containing messages to separated loved ones, foreboding messages of warning, and, most interestingly, past Survivors' own explanations for the Infection, varying from government conspiracy to an alien plot to an act of God.