"There is a virus sweeping through Grand Theft Auto IV turning many unsuspecting players into zombies," explains a release from publisher Rockstar.
Zombies are signified by a "zombie-with-yellow-briefs player skin." However, Rockstar warns that "the infected DON'T necessarily have to be wearing the zombie skin," as "some show NO SYMPTOMS!"
The virus is spread by killing an infected player in any multiplayer mode, and Rockstar has set up a page within its Social Club website to track the infection across the world.
Those seeking to begin the zombie outbreak will need the "Let Sleeping Rockstars Lie" achievement--earned by killing a Rockstar developer in a ranked match--before being able to choose the infected zombie appearance for multiplayer.