Arma II was released back in June 2009 but has been a top seller on the Steam games website for most of May, thanks to an update of the game based around zombies.
The Day Z expansion, or "mod", challenges players to survive in a game world suffering a zombie outbreak.
Survival involves scavenging food and medicines, and avoiding zombies and other players who stay alive by killing people and stealing their supplies.
Day Z has been created by Dean "Rocket" Hall, a developer for Bohemia Interactive, the game studio that makes Arma II.
He put the mod together in his spare time to test a few intuitions about what players wanted from a game. If his ideas proved correct and the test got good feedback, he planned to pitch it internally as an official game mod.
However, as he told the BBC: "That was before everything went crazy."