For anyone that doesn't want to click the link:
A new dungeon was opened in WoW recently, Zul'Gurub, which contains the boss Hakkar - God of Blood.
During his death throws, Hakkar uses a skill called Blood of Corruption, a contagion which infects players and kills low lvl characters instantly. Some griefers discovered they could infect their pets and take them into towns to spread the disease amongst the general population. This resulted in a virtual plague which has already claimed thousands of victims.