Corrupted Blood incident

The Corrupted Blood incident (also known as the World of Warcraft pandemic) took place between September 13 and October 8, 2005, in World of Warcraft, a massively multiplayer online role-playing game (MMORPG) developed by Blizzard Entertainment. When participating in a certain boss battle at the end of a raid, player characters would become infected with a debuff that was transmitted between characters in close proximity. While developers intended to keep the effects of the debuff within this boss's game region, a programming oversight soon led to the debuff becoming an in-game pandemic that spread throughout the fictional world of Azeroth.

World of Warcraft introduced the game region of Zul'Gurub on September 13. The boss of the region, Hakkar the Soulflayer, cast the debuff Corrupted Blood on raid participants, which expired when players defeated Hakkar. Corrupted Blood soon spread beyond Zul'Gurub through players deactivating their infected animal companions, who when reactivated in densely populated non-combat zones, still carried the debuff, becoming disease vectors, while non-player characters became asymptomatic carriers. Player reactions to the Corrupted Blood pandemic varied: some provided aid by healing players or warning them of outbreak zones, while griefers intentionally contracted the debuff to spread it across the game world. After several failed hotfixes, Blizzard ended the pandemic by performing a hard reset, and a later patch prevented companions from contracting Corrupted Blood entirely.