Just like the Scourge Invasion that preceded the release of the Burning Crusade expansion back in 2007, Blizzard has started to spread the plagued tendrils of the Lich King (their words) amongst its server population as a prelude to the arrival of the long-awaited expansion to the biggest MMO the world has ever seen.
Perks apparantly include zombification (does WoW support location based damage?), plagues and much much n00b griefing!
WoW Insider has the full scoop here.