In 2005, there was a major epidemic that affected a vast population of 6.5 million people worldwide, but it went largely unreported by the mainstream media.



That's because it was an in-game epidemic in Blizzard Entertainment's World of Warcraft that is now known as the Corrupted Blood Plague, or the Hakkar Blood Plague.



Even though the epidemic was virtual, it still provided fascinating insight into real-life epidemics, said Dr. Nina Fefferman at GDC 2011's Serious Games Summit.



Fefferman, an assistant professor at the Department of Ecology, Evolution, and Natural Resources at Rutgers University, explained, "World of Warcraft had at the time a really unique demographic composition. It wasn't the stereotypical gamer."



There were mothers, deployed soldiers, doctors, politicians, scholars -- people from all walks of life. That mix of people made the game the perfect venue to study behavioral trends amid an epidemic.



The Hakkar Blood Plague was a completely accidental incident. Blizzard at the time had introduced a high-level dungeon called Zul'Gurub, whose end-boss was Hakkar. He had an attack called "Corrupted Blood," which would drain hit points and make affected players highly contagious, and able to spread the debuff spell among other players.



Corrupted Blood was intended to be confined to Zul'Gurub -- Blizzard assumed that players would either die at the hands of Hakkar, which would rid players of the disease when resurrected, or that they would defeat Hakkar, which would also end the spell's effects.



But some players just ran away from the boss, taking their infected pets and infecting anyone they came in contact with. Lower-level player populations in particular were devastated by the plague. Some of those infected would even maliciously infect other guilds. "People were running around exploding into blood, dropping dead and turning into skeletons," said Fefferman.



Blizzard tried to impose a quarantine in order to contain the disease, which failed. Blizzard eventually had to reset the servers after about four of five days.



The way that players behaved in a way that a real-life population might react to an epidemic, said Fefferman. "We saw some courage... we saw some fear, suspicion of a quarantine," she said.



Blizzard's quarantine attempt also reflected how U.S. Homeland Security might react to a smallpox outbreak in a big city. And the fact that it didn't work in World of Warcraft could show how a real-life imposed quarantine might fail as well, said Fefferman.



What Fefferman learned from the Corrupted Blood Plague is that as ever, human behavior amid an epidemic is the wild card that can throw a wrench in even the most accurate, complex mathematical models that she and other epidemiologists use to predict the spread of disease.



"People are actually very difficult to predict," she acknowledged. "Behavior... that part is really hard."