The co-operative fight to keep the undead at bay has shifted over two and a half million copies at retail, with Steam sales figures unknown.
Valve keep their digital digits closely guarded, but they did reveal a not too long ago discount shot Left 4 Dead sales up 3000% at one point. The 2.5m includes both PC and Xbox 360.
Project lead Michael Booth made the reveal on left 4 Dead, the game released in November to huge success among fans and critics alike. Valve has even hired a psychologist to help them better understand the fickle mind of the consumer.