WoW Moviewatch: The Ironforge Plague
As far as Blizzard's design was concerned, this should have been it.
However, Blizzard overlooked a minor issue — pets could get Corrupted Blood. And pets could be dismissed while they had the debuff and then re-summoned into crowded areas. (The debuff was short-lived — so even if a player hearthed while under its effect, they wouldn't make it before it wore off. But when pets were dismissed, the debuff's duration was conveniently paused until they were re-summoned.) And the really fun part is that Corrupted Blood would spread to any nearby players. So, by this method, a few days in mid-September were spoiled for many a player in Azeroth, as Hakkar's plague went through the land, repeatedly infecting and killing players in major cities, while GMs tried to quarantine infected players and Blizzard scrambled for a hotfix.