The Corrupted Blood Incident was a game glitch that acted like a plague in the largest MMORPG, World of Warcraft. It occurred during the release of the Zul-Gurub Raid in Patch 1.7.
The final boss, Hakkar, released a debuff called Corrupted Blood, which dealt damage over time and could be spread to other players. While this debuff could be removed over time after a few seconds, it stayed on player's pets and minions and once players left the raid, the debuff spread across major cities like Ironforge, killing many lower level players. The debuff also spread to NPCs further spreading the debuff like an epidemic. Fortunately only one major server was affected before Blizzard quaratined the incident.
Due to constant complaints, Blizzard was finally able to put an end to the Corrupted Blood Incident with various patches and resetting the world servers. Because of this epidemic, real life epidemiologists became interested in the incident because of how this plague could resemble what could happen in the event of a real life outbreak.