If you're playing Grand Theft Auto IV multiplayer and suddenly find yourself surrounded by the undead -- you've just witnessed the special Halloween-themed viral content that Rockstar just announced to the public.

Niko! It's Roman! I want to eat your brains!

A note on the Rockstar Social Club site says: "If you've seen the special Zombie-with-yellow-briefs player skin running around during Grand Theft Auto IV multiplayer, then you may already know the virus can be contracted in any and all multiplayer modes and spreads when an unsuspecting player kills someone who has an infection."

Infection is not always detectable, so look for these clues to see if you've contracted a full-blown case of zombism: your character can access a zombie skin, and you'll score the new achievement "Let Sleeping Rockstars Lie."

Members of Rockstar Social Club, will be able to keep rack of how fast the infection has spread with a color coded map and how many have been infected in the past 24 hours.

Zombie sightings have been happening in Grand Theft Auto IV multiplayer for months now. YouTube user "FuzzyMcDoodles" records his transformation from Liberty City thug to zombie in the video below. Nice yellow Y-fronts, dude.

Been zombified yet? Do tell!