Corrupted Blood

Corrupted Blood was a virulent plague which originated in the dungeon of Zul'Gurub. It was unintentionally released into the larger game world, and became the first disease to affect entire servers.

Origin
Patch 1.7.0, released September 13, 2005, 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 demon players were stricken by a debuff called "Corrupted Blood" which would periodically sap their life. It inflicted 250–300 points of damage (compared to the average health of 4000-5000 for a player of the highest level at that time) every few seconds to the afflicted player.

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.

The disease would eventually go away as time passed or when the infected character died. The only way that a player was able to bring the disease outside of Zul'Gurub was by allowing a pet to get the debuff, dismissing the pet in less than five seconds, then summoning it in a populated area. (When dismissed, the pet retains the debuff and the timer of the buff is paused.) It caused problems because hunters dismissed their pets after being infected, and brought them out later at the stable masters in large cities. The disease could also be contracted by NPCs (who could also spread it outside of Zul'Gurub); due to this fact, the debuff quickly spread to large populations, instantly killing low-level players. Some players discovered that flagging themselves for PvP would prevent non-flagged players from infecting them; the players who were enjoying themselves by spreading the plague started flagging themselves as well (which eliminated the advantage of being flagged). 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 carpets of skeletons riddled the highest populated towns and 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 even influenced Blizzard's world event design for the Zombie Infestation event before the release of Wrath of the Lich King.

This was fixed in the 1.8.0 patch, as pets can no longer get the debuff, and the effect cannot exist outside of the Zul'Gurub instance.

Legacy
In late April 2009, Reuters published an article discussing the causes and effects of the Corrupted Blood outbreak, and its relation to the swine flu pandemic. University researchers studied the Corrupted Blood progression, and through it could more easily understand the spread of real-life diseases.

Although mechanics similar to the original Corrupted Blood was later used for other encounters, the original fight disappeared when Patch 4.0.3a (the pre-World of Warcraft: Cataclysm patch) removed the Zul'Gurub raid instance from the game.

Original outbreak

 * A video of a similar debuff being spread by a pet
 * Detailed information about that plague can be found here
 * Wonderland: Plaguelands..
 * BBC NEWS | Technology | Deadly plague hits Warcraft world
 * NPR story on real-world CDC interest in the plague (Nina Fefferman interviewed)
 * PC Feature: Looking Back... World of Warcraft - ComputerAndVideoGames.com
 * SecurityFocus|Digital Plague hits online game
 * New Scientist article: Virtual Outbreak
 * Corrupted Blood on Wikipedia

2007 research

 * FOX News: Online-Game Plague Could Help Scientists Track Real Epidemics
 * BBC NEWS | Health | Virtual game is a 'disease model'
 * Eric T Lofgren, Nina H Fefferman: The untapped potential of virtual game worlds to shed light on real world epidemics, http://infection.thelancet.com Vol 7 September 2007