Second Life Happy Farm World of Warcraft Ultima Online 
Answer: World of Warcraft
In the fall of 2005 things got a little out of hand in the virtual world of the wildly popular massively multiplayer online role-playing game World of Warcraft. Specifically, a new dungeon was introduced that included an end boss who, when attacked, cast a spell on the characters called “Corrupted Blood”.
The spell was intended to only last a few seconds and to function only within the area where the players were fighting the dungeon boss. Unfortunately, a glitch in the game extended the duration of the spell and made it transmissible, like a virus, to other players. Players could teleport outside of the dungeon and, in doing so, bring the virus outside.
The infection spread rapidly as players accidentally (and, in some cases, maliciously) spread the virus. Players fled highly populated areas, formed refuge groupings of non-infected characters, and otherwise altered their virtual behavior to avoid infection. Some characters with healing abilities became virtual medics helping the infected, others directed players to safe havens, and some were unable to resist purposely spreading the disease.
The entire range of behaviors and the transmission models so closely mirrored the real world, epidemiologists quickly descended on the virtual world to study the behaviors of the players and the spread of the infection. By studying the incident, the epidemiologists were able to monitor all sorts of real world behaviors like altruism in the face of outbreaks, the effects of bystander-curiosity on infections (many players were unable to resist checking out the hot-zones in the game and became infected as a result), and other elements of outbreak movement and psychology. Many researchers have stated how valuable the entire incident was in terms of research progress because the logistics of building a research model driven by millions of actual human inputs would be prohibitive in every way and conducting such research in the real world with a planned outbreak would be unethical and immoral.
Eventually, through a series of game patches, quarantines, and rollbacks, the Corrupted Blood virus was brought under control.