World Of Warcraft Girl

The Corrupted Blood plague incident was one of the first events to affect entire servers. Patch 1.7 saw the opening of Zul'Gurub, the game's first 20-player raid dungeon where players faced off against a tribe of trolls under the sway of the god Hakkar the Soulflayer. Upon engaging Hakkar, players were stricken by a debuff called "Corrupted Blood" which would periodically sap their life. The disease was also passed on to other players simply by being near infected players. Originally this malady was confined within the Zul'Gurub instance, but it made its way into the outside world by way of hunter pets or warlock minions that contracted the disease.
Within hours, Corrupted Blood had completely infected major cities because of their high player concentrations. Low-level players were killed in seconds by the high-damage disease. Eventually Blizzard fixed the issue so that the plague could not exist outside of Zul'Gurub.
The Corrupted Blood plague so closely resembled the outbreak of real-world epidemics that scientists are currently looking at ways MMORPGs or other massively distributed systems can model human behavior during outbreaks. The reaction of players to the plague closely resembled previously hard-to-model aspects of human behavior that may allow researchers to more accurately predict how diseases and outbreaks spread amongst a population.