If this plague sounds familiar, we'd direct you towards this post from 2005 (lo those many years ago) detailing an unintentional plague that spread through Azeroth (and the servers that power it). Blizzard has, in short, taken that "Blood Plague" and harnessed it as a world event, similar to the Scourge invasion that preceded the release of The Burning Crusade. Unsurprisingly, WoW Insider has all the details, reporting, "The threat is growing -- we're getting word of huge zombie raids forming outside the capital cities ..."
Shhh. You had us at "huge zombie raids."
Gallery: Wrath of the Lich King Zombie Event