Got this from Sam. This was news to me:
In the last week, it added the Zul’Gurub dungeon which gave players a chance to confront and kill the fearsome Hakkar – the god of Blood.
In his death throes Hakkar hits foes with a “corrupted blood” infection that can instantly kill weaker characters.
The infection was only supposed to affect those in the immediate vicinity of Hakkar’s corpse but some players found a way to transfer it to other areas of the game by infecting an in-game virtual pet with it.
This pet was then unleashed in the orc capital city of Ogrimmar and proved hugely effective as the Corrupted Blood plague spread from player to player.
Although computer controlled characters did not contract the plague, they are said to have acted as “carriers” and infected player-controlled characters they encountered.