Blood Plague

The Corrupted Blood incident 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.