When it comes to a global pandemic, human beings are the ultimate wild card. That makes it challenging to build accurate mathematical models to predict how the progress of the disease will play out. We've certainly seen plenty of all-too-human responses to coronavirus over the last two weeks, with some people panicking and hoarding food, toilet paper, and hand sanitizer. Others cling to denial, and still others are defying calls for "social distancing" by continuing to go to restaurants, bars, concerts, and so forth. Our epidemiological models are a bit better able to account for that unpredictability thanks in part to a virtual outbreak in World of Warcraft nearly fifteen years ago, known as the "Corrupted Blood incident."

The Corrupted Blood outbreak was not intentional. In 2005, Blizzard Entertainment added a new dungeon called Zul'Gurub into World of Warcraft for highly advanced players, controlled by an "end boss" named Hakkar. Hakkar was a blood god known as the Soulflayer, who had, among his arsenal of weapons, a "debuff" spell called "Corrupted Blood." Infected players would suffer damage at regular repeating intervals, draining away their "hit points" until their avatars exploded in a cloud of blood. The only cure was to kill Hakkar.

Blizzard thought this would ensure the infection wouldn't spread beyond that space. They were wrong. Rather than standing their ground, many infected players panicked, teleporting out of the dungeon before dying or killing Hakkar, and taking the disease with them. And lower ranking players, with fewer hit points, would "die" very quickly upon exposure.

The biggest factor in the rapid spread of the disease was a glitch in the programming, such that non-playable animal companions also became infected. They didn't show symptoms, but they were carriers and ended up spreading the disease even faster. As Corrupted Blood infections spread uncontrollably, game spaces became littered with virtual "corpses," and players began to panic. Efforts at quarantine proved unsuccessful in stopping the outbreak. In the end, at least three servers were affected, and Blizzard had to reboot the entire game to correct the problem.

An epidemiologist named Eric Lofgren, then at Tufts University, just happened to be an avid WoW player and was fascinated by the real-world parallels to how the epidemic played out in the virtual world. He and his Tufts colleague, Nina Fefferman, co-authored a 2007 paper published in Lancet Infectious Diseases examining the potential implications of the Corrupted Blood incident for refining existing epidemiological models, since they would be able to draw on hard data showing how players actually responded during an outbreak.

For instance, some players tried to help with healing spells, inadvertently making matters worse, since their efforts endured constant replenishment of those susceptible to the spell, rather than letting the outbreak run its course. There were the inevitable thrill seekers who went to the infected areas out of curiosity, becoming new victims, which Fefferman has likened to journalists traveling to war zones who put themselves in harm's way to get a story. There were a handful of players who maliciously spread the infection on purpose—something that has been documented in real-world outbreaks—and one player even took on the role of a Doomsday prophet, standing in the town square to narrate the carnage unfolding in the game.

Fefferman is now at the University of Tennessee, Knoxville, where she studies how tiny individual decisions can result in major changes across a given population. That earlier work on the Corrupted Blood outbreak continues to inform her research.

"It led me to think really deeply about how people perceive threats and how differences in that perception can change how they behave," Fefferman recently told PC Gamer. "A lot of my work since then has been in trying to build models of the social construction of risk perception, and I don't think I would have come to that as easily if I hadn't spent time thinking about the discussions WoW players had in real time about Corrupted Blood and how to act in the game, based on the understanding they built from those discussions."

Lofgren is now at Washington State University, a US region particularly hard hit by coronavirus. His current work focuses on the stresses these kinds of outbreaks place on the healthcare system.

"For me, it was a good illustration of how important it is to understand people's behaviors," he recently told PC Gamer about the earlier WoW study. "We often view epidemics as these things that sort of happen to people. There's a virus and it's doing things. But really it's a virus that's spreading between people, and how people interact and behave and comply with authority figures, or don't, those are all very important things. And also that these things are very chaotic. You can't really predict 'oh yeah, everyone will quarantine. It'll be fine.' No, they won't."

So it's not surprising to witness people behaving in very similar ways to the current pandemic as the WoW players did back in 2005. "Corrupted Blood was this unexpected black swan event. We treat this [coronavirus] as if it's unexpected, but nature is really good at getting people sick," said Lofgren. "If you think again in gaming terms, we're making saving throws against new emerging diseases all the time. And sometimes you fail. We have epidemics recur with some degree of frequency. It's sort of like getting rid of people who predict earthquakes because you haven't had an earthquake in a while. Well, yeah, you're gonna have another one."

DOI: The Lancet, 2007. 10.1016/S1473-3099(07)70212-8 (About DOIs).