Play any GTA IV co-op lately? No? You might want to head back into Liberty City then. There's been some changes around town, including a mysterious virus that is changing players into zombies. Brain eating, blood spewing, Russian gangster zombies at that.
The disease exists in any of the multiplayer modes and occurs when a player kills another player who is infected. What's worse? That player may not even KNOW they are infected yet. Players that are infected will either have the "Let sleeping Rockstars Lie" achievement, or be sporting the zombie player skin.
You can track the spread of the virus at Rockstar's Social Club website. All the more reason to play co-op and not kill each other, right? Nobody wants to BE a zombie, they only want to kill them.