On Sept 13, 2005, a disease began to spread in the massively multiplayer online role-playing game (MMORPG) World of Warcraft.

An update to the game had introduced a new area for higher-level players, known as Zul'Gurub. In the area, there was a boss called "Hakkar the Soul Flayer" (warning: this article may contain words used by dorks), a massive winged serpent that had a power that hadn't been introduced to the world yet: "Corrupted Blood".

The idea was that Hakkar would infect players with the disease, dealing 875-1125 points of damage plus 200 damage per second to the target and all nearby enemies for 10 seconds. The unique part of this disease was that it would spread to other players in the vicinity of the infected player, all of whom would then lose health.

It was supposed to be annoying to players during this one battle, but ultimately stay in this specific area while you fought the boss. However, the creators hadn't thought things through.

Hakkar was able to transmit the virus to players' pets. Unfortunately, this meant that when you dismissed your pet when they were infected, unknown to patient zero, they remained infected until they were summoned again somewhere outside of the boss area. 

Patient zero summoned his pet in a city elsewhere in World of Warcraft, and the plague spread like, well, a plague. Adding to the problem was that non-playable characters (NPCs) that give out quests to the players could contract the disease and spread it to others, without taking damage or dying, essentially acting like asymptomatic spreaders.

The resulting pandemic became what the players (and epidemiologists) now refer to as the Hakkar Blood Plague or the Corrupted Blood incident, footage of which can be seen below.

What's especially cool about the incident, especially for epidemiologists, is how well it modeled actual viruses and outbreaks in the real world, as has been pointed out in a thread recently on Twitter, as well as in a 2007 paper published in The Lancet.

It killed low-level players quickly, simulating the vulnerable such as the elderly or the immunocompromised. You have players who can teleport from city to city, simulating traveling by air in the real world. Stronger players could withstand the disease, barely being weakened by it, simulating those who get mildly ill from a disease.

Then you have the added bonus that people would behave in ways similar to how they would act in the real world. 

In an attempt to control the outbreak, the game's makers tried to instigate a quarantine, asking the infected to isolate themselves from uninfected areas. As with real life, many players were compliant but others resisted the quarantine. Due to the highly contagious nature of the disease, the outbreak soon spread through the cities.

As well as people who would spread (on purpose or by accident) the disease around cities, you also had many people who would attempt to heal the sick and the dying, acting like first-responders and medics in the real world. According to the Lancet paper, unfortunately, this may have extended the Corrupted Blood outbreak "by keeping infected individuals alive long enough for them to continue spreading the disease, and by becoming infected themselves and being highly contagious when they rushed to another area" though overall they may have reduced mortality rates.

Ultimately, attempting to deal with the outbreak through in-world measures failed due to players not taking the risks seriously, and heading to areas where there were infections, spreading it themselves to others. It had to be fixed by rolling out another update.

Though slightly silly and ultimately of no consequence, it was a good model to see if an extremely highly-contagious disease could be contained through public health actions. Many of the players' commitment to the game was so strong that it gave researchers a pretty good model for how people would act in a real outbreak.

"All together, these seemingly innocuous aspects of the game world, each directly mirroring an aspect of real-world epidemiology, allowed what should have been a very minor point of interest in a small area of the game, catering to a very specific subset of the players, to become the first online instance of uncontrolled plague to affect millions of Americans, Asians, and Europeans at home," authors of the 2005 study wrote in their paper.

It gave some good lessons for how people might deal with an outbreak in the real world, and the authors made suggestions for how further games could be used to model more realistic diseases that might cause a pandemic.

Then, unfortunately, we all learned the hard way.