The Corrupted Blood outbreak occurred after the game's designers created a virus to infect players battling an enemy in a specific dungeon. They designed the virus to spread to other players in the same region. The virus eventually spread from the dungeon, which was supposed to contain it, and ultimately decimated entire cities in the game. And here is where the player behavior is fascinating: During the pandemic some players tried to combat the virus by healing other players, but other players actually spread the virus maliciously.
Now health researchers are studying the incident because it may help them understand how people will react to a pandemic:
Researchers requested data from Blizzard, the game developer, so it could be analyzed and at least two articles have appeared in peer-reviewed journals about the incident. One in Epidemiology and another in The Lancet Infectious Diseases.