Left 4 Dead's sales continue to rise, proving that a game with no greater premise than shooting zombies can be very successful. By current count, the project lead for the game claims L4D has now sold 2.5 million copies in retail since it first was available on store shelves late last year. That doesn't include sales from Steam, though, as these figures are not publicly available.

What are the total sales figures for the game? Only Valve and Turtle Rock know for sure, but we can assume it's an ample number given how much Valve advertised Left 4 Dead on Steam and how well the Steam platform in general has been doing. On top of that, in the middle of last month there was a thirty-fold increase in Left 4 Dead sales during a brief half-price reduction on the game.

The Left 4 Dead DLC packages, which Valve promises to keep free, certainly help these figures as well. It would be nice, though, if they've cough up some actual numbers.