Corrupted Blood plague
Main article: Corrupted Blood
While not an intentional world event, the Corrupted Blood plague nonetheless was one of the first events to affect entire servers. Patch 1.7 saw the opening of Zul'Gurub, the game's first 20-player raid dungeon where players faced off against an ancient tribe of jungle trolls under the sway of the ancient Blood God, Hakkar the Soulflayer. Upon engaging the Blood God, players were stricken by a debuff (a spell that negatively affects a player) called "Corrupted Blood" which would periodically sap their life. The disease would also be passed on to other players who were simply standing in close proximity to an infected person. Originally this malady was confined within the Zul'Gurub instance but made its way into the outside world by way of hunter pets that contracted the disease.
Within hours Corrupted Blood had infected entire cities such as Ironforge and Orgrimmar because of their high player concentrations. Low-level players were killed in seconds by the high-damage disease. For days skeletons carpeted the highest populated towns, which were rendered uninhabitable by the persistent plague. Eventually Blizzard fixed the issue so that the plague could not exist outside of Zul'Gurub but fan reaction to the epidemic was strangely positive and may influence Blizzard's world event design for the future.
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