After first being announced in July, Social Club's "Viral Infection" zombie feature is now finally available at Rockstar Social Club. Here is the description from Rockstar:

There is a virus sweeping through Grand Theft Auto IV turning many unsuspecting players into Zombies. 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 that the virus can be contracted in any and all multiplayer modes and is spread when an unsuspecting player kills someone who has the infection (note that the infected don’t necessarily have to be wearing the Zombie skin, some show no symptoms!). The signs of infection are: access to the zombie skin and having the Achievement “Let Sleeping Rockstars Lie”. With the all-new Viral Infection feature at the Rockstar Social Club, members can now see whether they are currently infected, the number of total global infections and the number of infections in the last 24 hours. Digging deeper, Social Club members can also browse a color-coded world map tracking the spread of the infection worldwide, including breakdown for all 50 US States. Click here for the Viral Infection feature.

Link: Rockstar Games Social Club