The PC version of the game, or at least part of it, has been circulating among pirates in the U.S., who illegally copy and share it without paying Activision Blizzard or its retailers. That’s a big problem, as Activision Blizzard is expected to sell more than 20 million copies of the game in 2011 alone, according to Arcadia Research. That amounts to more than $1.2 billion in sales at retail.

Piracy has always been a problem with video games, especially for PC titles. But the pirates go after Call of Duty because it’s the big prize. Every year, a cat-and-mouse game ensues. Activision Blizzard closely monitors manufacturing in the weeks leading up to shipment, since it only takes one stolen disk to set off the viral spread of pirated copies. But it’s hard to stop inside jobs.

Evidently, pirated discs began to spread quickly after an employee at a shipping warehouse (run by a huge shipping company) in Fresno, Calif., stole one disc. It’s not clear how many full copies spread this way, as the disc that was stolen was apparently Disc 2 of a two-disk set for the PC. Security may have been too tight for the thief to steal the other disc.