The Corrupted Blood plague is quickly spreading across World of Warcraft servers. The disease originates from the recently added instance, Zul'Gurub. Players that go up against Hakkar (the god of blood) may find themselves infected with its debuff, Corrupted Blood, which does roughly 250-350 damage and can affect nearby players. Some of the players that have survived their battles with Hakkar have brought the disease into towns, spreading it amongst the general population, including npcs. Be warned.
Evil Avatar has a video of the plague in action.
[Thanks, Jaime T.]