Just in time for Halloween Zombies are beginning to run rampant in Liberty City. As seen on the Rockstar Games Social Club, players can now be infected in any multiplayer mode in GTA:IV. An infections occurs when "an unsuspecting player kills someone who has the infection." There is a special zombie skin that is unlocked when you are infected that can let people know that you are a zombie, but beware not all of those infected are wearing the skin.
Through the Social Club you can track the spread of the infection across the United States and around the world, as well as seeing your current infection status. So beware when running through Liberty City, you could just end up a zombie!