Blizzard's latest patch for World of Warcraft had a rather unintended and unfortunate effect when it was implemented recently. One of the additions to the game under the patch was a new instance, Zul'Gurub, which contained a monster with a rather unusual ability: an attack called "Corrupted Blood" that infected players with a disease that knocked off a fairly high amount of HP every 2 seconds.
Now, this wouldn't be such a big deal, ordinarily - many of the game's monsters have a variety of devastating attacks - but what made this attack particularly troublesome is the fact that the infection caused by the Corrupted Blood could be spread to both other players and NPCs. Needless to say, such a plague was a disaster waiting to happen, and happen it did; all it takes is for one gamer to return to town while still infected for the plague to wreak havoc on a server's population.
Many of the game's servers have experienced such an unfortunate effect as of this writing, and play has become impossible by reason of the plague spreading rapidly across affected servers' towns. Corrupted Blood doesn't cause particular difficulty for high-level characters, but for the NPCs and lower-level characters, it poses a real problem, and most towns and cities in affected servers have become plague-ridden graveyards.
Blizzard GMs attempted to solve the problem by quarantining some infected players, but regrettably, malicious gamers have always been a problem on Battle.net, and so inevitably, some of the infected players escaped the quarantine and continued the cycle of infection.
At this time, there does not appear to be any information on the progress of the fight against this problem. For more information on World of Warcraft, visit the official site.