On September 13, 2005, Blizzard added a raid to World of Warcraft with a boss, Hakkar the Soulflayer, who cast a debuff called Corrupted Blood that spread between nearby players. It was only meant to last inside that one dungeon. A programming oversight let it escape into the rest of the game world instead.
Players fast-traveled home from the raid while still infected, then reactivated the pets they'd put in stasis during the fight — and those pets carried the disease into crowded cities. Within days, Azeroth's capitals were, in one player's words, "filled to the brim with corpses," the "streets literally white with the bones of the dead." Some players tried to heal the sick or warn others away. Others deliberately caught the plague and ran into crowds to spread it; one told Wired, "It's just funny to watch people run away screaming."
Blizzard tried quarantine zones and server restarts, but the plague kept reappearing, and after a week the company hard-reset the servers to their pre-outbreak state. The CDC reached out hoping to study it as pandemic data; Blizzard had to explain it was an accident, not a planned simulation. Researchers still found it useful, since human responses in the game added "a level of authenticity that doesn't exist in other simulations."
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