What 2005 virtual plague in World of Warcraft drew interest from epidemiologists?

The 2005 virtual plague that attracted epidemiologists was the Corrupted Blood incident.

The outbreak began on September 13, 2005, when Blizzard introduced Zul’Gurub, a raid zone whose boss, Hakkar the Soulflayer, could infect characters with the Corrupted Blood debuff. The effect was intended to remain confined to the encounter, but infected hunter and warlock pets could carry it outside the raid. When those pets were dismissed and later resummoned in crowded cities, the disease spread rapidly.

From September 13 to October 8, Corrupted Blood became an unintended pandemic across Azeroth. Some players healed victims and warned others away from infected cities; others deliberately spread the debuff. Blizzard eventually contained the incident through emergency measures, server resets and a patch that stopped companion animals from carrying the disease.

Researchers found the episode interesting because player behavior—travel, crowding, avoidance, cooperation and deliberate transmission—resembled social factors in real outbreaks. It was not a true epidemiological simulation, however: characters could recover or resurrect, and the virtual disease had unusually high transmissibility.