A week after the outbreak, it forced Blizzard to restart every WoW server to stop it from spreading out of control.

The infection originated from Hakkar the Soulflayer — the boss of the very first 20-player raid Blizzard released. Hakkar would cast Corrupted Blood on players and it would damage them for about 10 seconds. Players would spread the effect to others if they got too close to those infected. After the 10 seconds were done, or players finished the boss battle, the damaging effect was supposed to end. Only it didn’t.

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A programming oversight allowed the debuff to spread beyond the site of the Hakkar boss fight and into the world at large. Much like rats fueled the Bubonic Plague, characters’ trained animals spurred the Corrupted Blood outbreak. Hunter characters can summon and dismiss pets to fight at their side at will. Once dismissed, all the effects on the pets are paused until it’s called back out again. In effect, the pets would contract Corrupted Blood during the boss fight, disappear and then exhibit the symptoms again elsewhere in the world map when they were again summoned. There it would spread to other players and pets that came in contact with them.

Cities like the dwarven city Ironforge and orc city Orgrimmar were overrun within hours. Non-playable characters, who couldn’t die due to special coding, would also catch the effect, meaning any player who passed by them could receive Corrupted Blood.

Once word got out, players searched frantically for news about what was going on.

“The world chat would explode any time a city fell,” says Nadia Heller, an ex-World of Warcraft player whose character lived through the incident. “We kept a close eye not only on our guild chat but on world chat as well to see where not to go. We didn’t want to catch it.”

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The spread of Corrupted Blood, and the player’s behavioral changes to it, caught the attention of epidemiologist Dr. Nina Fefferman, who was a World of Warcraft player at the time of the incident. Fefferman reached out to her colleague Dr. Eric Lofgren. In 2007, the two published a paper that detailed their findings, including complex models of human behavior during a pandemic. Fefferman says the incident has helped inform her current research into predictive modeling around covid-19.

“What I do is study all the aspects of infectious disease outbreaks that help us prepare for pandemics,” said Fefferman, a mathematical biologist. “We really saw the full gamut of behaviors we see in the real world reflected in the player characters during Corrupted Blood.”

Dr. Dmitri Williams, an associate professor from USC who was also playing World of Warcraft during the Corrupted Blood incident, questions whether Fefferman’s findings are valid mirrors to real-life behavior.

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“There are games where you are encouraged to behave in a way that you would never behave offline,” Williams said. “You really have to know [the game], play it and understand the culture so you can make these kind of determinations that, yeah, this is a pretty good proxy.”

Despite this, Fefferman believes that virtual worlds like World of Warcraft are perfect testing environments for mass behavioral reactions to outbreaks.

“It’s not just that people were role-playing. People were being themselves,” Fefferman said.