Back in 2005 there was an epidemic that killed millions. It was 100% contagious but not 100% fatal. There were hundreds of thousands, perhaps millions, of deaths…but it’s hard to figure out the actual death rate because it could kill people over and over again. It occurred in World of Warcraft (WoW). It’s known today as the Corrupted Blood incident, and it’s been used to better understand human behavior during an epidemic.

Where were you when Corrupted Blood happened?

At the time it began to really be an epidemic I was only lvl 54 with my little 40% mount. Trolling 60s with the plague would chase us slower players until we contracted [it] and died. Those trolling players conducted mass witch hunts for lower levels hiding throughout zones. I once ventured into a mine seeking refuge and all the way in the back I saw other low levels huddled. It was sickening how real it was.

JB, commenting on Player Base’s YouTube channel

I wasn’t playing WoW back then (this was only a year after WoW was released); I was primarily the new mother of an infant and living in Athens. But here’s the tale as it’s been related to me.

Sucks to be first level

One year after game released, WoW boasted almost 2 million players and could have as many as 500,000 people logged in at once from all over the world. On September 13, 2005, Blizzard (the company behind WoW) released patch 1.7.0, which included a new 20-person raid called Zul’Gurub.

(I assume everyone knows gamerspeak by now, but just in case: A player creates a character to interact with the game. That’s a “player character” or PC. You start at 1st level and get more powerful by succeeding at the challenges in the game, which award you Experience Points. In WoW, most Experience comes from killing mobs in the open world and dungeons, completing quests and bonus objectives, and finishing dungeons. You can’t do all this alone, so you team up with other PCs. Guilds are an emergent element of WoW and other MMORPGs that make grouping and teaming easier and provide social interaction and cohesion. The game developers also design specific events by adding new areas to the map complete with enemies and loot. These are called instances. Raid instances require playing as a team, and are designed to be the most challenging and entertaining PvE content available in the game. Members of a guild will self-assemble to form a raid group and go into the raid instance. This all requires coordination, which requires communication, so you have voice and text options.)

WoW has some…overtones. While all the playable races are based on European mythology, the cultural trappings used to be pretty clearly divided between “European = Good” and “Not European = Not Good.” For instance, trolls – a Nordic myth IRL – are primitive and tribal in the game, speak with African or Caribbean accents, and, back then, lived in Central American Mayan style areas. (They still do, but there are sand trolls and ice trolls and stuff now. FWIW.) Zul’Gurub was in a pseudo-Central America, with pyramidal ruins poking out of jungle and vine bridges. It was a really tough raid; groups had to be 60th level to get anywhere with it.

(WoW, as well as other group-based RPGs, are excellent teaming simulators. Rather than take the approach of many workplaces, i.e, “we’ll tackle this crisis with whoever’s here,” gamers think about the roles and capabilities they need, and how they complement each other. In WoW, you want the “trinity:” the right balance of Tanks, DPS, and Healers. Tanks are PCs that can absorb a lot of damage, and their role is to draw attacks away from the DPS and Healers. DPS stands for Damage Per Second; these are the PCs that deal damage to the opposition, but they can’t withstand as much punishment as Tanks. Healers are there to keep the Tanks and the DPS up and functioning until victory has been achieved.)


The final battle was against Hakkar the Soulflayer, also known as Hakkar the Blood God. He’d been summoned by an evil troll (I mean, they are all evil villainous members of the Horde). Haakar appeared as a giant feathered serpent and had magical as well as physical attacks. One of his abilities was a debuff

(More gamer talk: “to buff” is to temporarily make a character stronger or more capable through magic; “to debuff” is the opposite. A buff is a positive effect, like additional health or strength. A debuff is a negative status effect that either strips a buff or causes a new negative effect. One type of negative effect does DOT (Damage Over Time), meaning, while it’s on your PC, your PC takes damage from it automatically. It’s essentially like having an illness.)

called Corrupted Blood. Corrupted Blood immediately did about 1000 pts of damage to the PC it hit, and then did 200 points every second for 10 seconds. To jazz it up a little, it was contagious = if the infected PC was near another PC, Corrupted Blood would jump to the new PC and start doing damage…even if the new host had already had it and survived. At the time, a well-geared raid Tank had a max health of 4000-5000, so you can see that Haakar was a raid boss designed for the best high level groups. Lower level PCs wouldn’t even make it to him, so the fact that one hit of Corrupted Blood would immediately kill a low or mid-level character was moot. In fact, Corrupted Blood was a clever debuff to overtask raid Healers and to wipe out poorly designed raid groups or teams that didn’t coordinate well. Only the best were supposed to make it through.

screencap of party fighting Haakar…and someone dying

And the best did! I looked for video and sound of the Corrupted Blood effect, but couldn’t find it. Picture blood exploding out of your character accompanied by a SPLURTCH! Players quickly realized that as soon as they contracted it they needed to run away from their group lest they give it to their teammates. Healers kept an eye on the HUD and would tell infected players to RUN, following them to heal them. It was a lot like ebola, without the fomite spread.

After a long and grueling battle, the PCs would teleport back to the open world, almost always to one of the large cities, to meet up with the rest of their guild, sell stuff, resupply, etc. and mingle with PCs of all levels.

And then those characters started dying. Blood explosion-SPLURTCH!-skeleton collapsing to the ground. Corrupted Blood had escaped the raid instance and tore through the densely populated cities.

That whole week we referred StormWind and Ironforge as “the contaminated zone” because players and NPCs were dropping like flies.

From Wuhan wet market to Lunar New Year celebrations

i remember this so clearly. The corpses littering Durotar all the way to Thunderbluff and undercity. death was everywhere except the wilderness. luckily i was maxed out and tried to help as much as possible, i actually felt like a doctor and loved playing Priest.


People started fleeing the cities…but they brought the plague with them, along the major thoroughfares and into the rest of Azeroth. Players realized what was happening and attempted to quarantine, locking the cities down and warning people not to enter while trying to keep the infected from leaving.

We called our selves the Burning Phoenix Brotherhood, we weren’t a guild we were just a 6 man squad of a bunch of level 20s. I remember we just got done a multi step quest and were heading back to stormwind to collect our prize. We ran into some one who was running away from stormwind and told us what happened. We took shelter in the Mine shafts and kept killing the same enemies over and over and over again for days.

Decepticons and Cobra, commenting on PlayerBase’s YouTube channel

Healers ran to the cities, setting up treatment centers and trying to heal the infected. Only the highest-level Healers could withstand it; weaker Healers died. Of course, they could respawn and heal more PCs but every respawn makes you weaker still…and unfortunately, healing a PC meant that that PC could spread the contagion.

dude it was so amazing healers at the frontvof citys curing pepoul so amazing and i was helping

(basically all these quotes are from YouTube comments)

when the event was happening, me and my guild mates tried to set up healing stations during the beginning of the problem but it soon became apparent that it was hopeless. Since the patrol npc’s could carry it and since it never did enough damage to them to kill them, they would wander the cities and spread it constantly. Even if we managed to fix one area, it began to pop up elsewhere and it simply became too much to deal with. After that, as interesting as the “event” was, the novelty of it wore off after a few days and it began to be quite irritating.

Ironically, the dungeons of Azeroth became some of the safest places to be (along with the uninhabited area called the Plaguelands). Many players abided by the quarantine (all you really had to do was NOT LOG IN) but many players did not. Some didn’t know they were infectious, others tried to run from it…but some PCs who were strong enough to carry the plague and not die from it purposefully broke quarantine, chasing down lower level characters to infect and kill them.

It was fun, back when i played WoW i was one of the horde players that had it on my pet. I use to spread it around on alliance, it was honestly funny shit i was so sad when it was patched 😂😂😂

This is known as trolling or griefing (not “grieving” – grieving is to feel grief while griefing is to cause grief and if you like this sort of language stuff may I recommend Because Internet by Gretchen McCulloch) while the term comes from virtual worlds, the behavior happens IRL also.

WoW players responded faster than Blizzard did, with some taking on the roles of journalist or Doomsday prophet, “yelling” in world chat or otherwise trying to make sure all players were up to date. Blizzard tried to impose restrictions on player behavior and did rolling restarts of the affected servers, but nothing worked. One week after the Corrupted Blood pandemic began, Blizzard shut down all WoW servers and patched the game with v1.8, which solved the problem.

I remember playing a lol 25-30 gnome Warnock and I was about to fly to Ironforge, when I saw text in the General Chat warning any low lol people to stay out of the cities and starter zones as well due to the infection. I took shelter in Talondeep Pass because my hearthstone was set to Ashenvale and I camped there for 2 weeks following the first signs of outbreak.

How did this happen?

Corrupted Blood, like covid-19, was zoonotic. It jumped from an animal (Haakar) in the wilderness to a “human” PC. The game coding scrubbed it from PCs when they left Zul’Gurub…but animals remained infected.

Some classes of PC have pet animals. Hunters and Warlocks can summon their pets to fight with them in combat. As part of the class’ suite of skills, these animals are subject to same debuffs, such as Corrupted Blood. After fighting Haakar, players dismissed their pets…but the debuff was still active on them. Healers didn’t think about healing the pets before teleporting. Groups returning from Zul’Gurub would often head to cities with high population densities like Ironforge or Stormwind, where the Hunter or Warlock would summon their pet again, and Corrupted Blood would move from the pet to all the nearby PCs.

honestly I spread this by mistake, I dismissed my pet in the raid holy shit that was horrible.

It infected NPCs as well. Innkeepers, auctioneers, and other characters are permanent residents and are necessary presences. Therefore, Blizzard had coded them to not be killable (because otherwise OF COURSE some players would kill them). These NPCs are epicenters of player activity, so once infected they becaming undying superspreaders.

Memories fade after 15 years and witnesses don’t agree; some say the NPCs like the pets and innkeepers were asymptomatic – they didn’t have the blood splatter and SPLURTCH! animation. Either way, they did their part to escalate the impact. When Blizzard fixed the game with Patch 1.8, they made it so that pets couldnd’t get the CB and CB couldn’t exist outside Zul’Gurub.

I only wanted to check out this blood pleauge for myself however I did not get to see much just a band of infected trollers who would run around trying to infect a non infected town or hideout or mass group of players it was like 3 level 6’s with really low HP I shoot them with my bow and ya… then the pach came in.

Demiclea on Extra Credit’s YouTube channel

Turns out epidemiologists play WoW

Jep. Only lasted 10sec. It also cleared on death. Wich makes it more interesting because you would think that the plague would fizzle out once enough people died or got over it. But reinfections and constant moving in of respawns, unknowing players and even gawkers kept it alive. This crowd behavior made it a living moving system, which is why scientists even cared.

Dr. Eric Lofgren was a World of Warcraft player at the time of the incident, as well as an epidemiologist at Tufts. Lofgren immediately called his research partner, Dr. Nina Fefferman, and together they started working to learn as much as they could. This had to be largely ethnographic observation of in-game behavior, as Blizzard wasn’t keeping the kind of data that the scientists needed to understand how and why the epidemic spread.

I got mine from an orc hunter who was waiting at the zepplin to orgrimar. I ran around looking for a paladin or priest to cleanse me, not realizing that my damn voidwalker was bouncing the plague to me and everyone else. As I rushed to Undercity looking for help i yelled in general chat for help, i had just spent the last of my gold trading for an item from nefarians lair. I ran around draining life from wolves to survive but it wasn’t enough, they died in one shot. I must have accidentally killed at least 30 people trying to find a priest in Undercity. It’s fascinating what thoughts run through your mind when you fear the repair bill. RIP all those noobs i killed in undercity, your sacrifice wont be forgotten.

Some of what they saw tracked with the current science around epidemics…but some did not. It turns out that, much as economists used to assume humans were rational actors for their economic models, epidemiologists used to assume humans would be rational actors.

For instance, Fefferman and Lofgren were both curious about the disease, and so exposed their WoW characters to it to see what was going on. “That’s something we [had] never put into the epidemiological model,” Fefferman says. While it might seem unrealistic for people to put themselves at risk of a disease out of curiosity, Fefferman says it’s not all that farfetched. Journalists, for example, might get closer to an outbreak than they should to find out what’s going on. So might public health researchers. 

Griefing was another behavior that wasn’t in the models. When they published their study on the Corrupted Blood Incident in the Lancet in 2007, Lofgren and Fefferman translated greifing as analogous to someone going into work knowing they are sick, but during Covid-19 we’ve seen much more greiferlike behavior. People are often much more asinine than you expect. They are also often much more philanthropic; models didn’t account for healthy people going into infected areas to help if they weren’t medical staff.

It was quite fascinating actually, I was a 60 level priest at that time and I saw it spread ever since the beginning, I was in a group of healers that was going to cities, trying to heal everyone, but it wasn’t really helpful, so I gave up and went with the non-infected members of our guild (literally) camping, all of them were lvl 40 or lower so I felt like their mum, lol. Good times, I know it wasn’t intentional, but it just made the game seem real to me, seeing how other players reacted – hiding away somewhere safe and higher levels actually trying to help others survive,

In pandemics as in economics, 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…and while Kahneman and Tversky were able to design and run experiments that helped them create the field of behavioral economics, it’s both difficult and unethical to cause a disease outbreak in order to study human responses.

Thus, we need to take maximum advantage of any real outbreaks, like covid right now. We’ve certainly seen plenty of all-too-human responses to coronavirus, like people panicking and hoarding food, toilet paper, and hand sanitizer. We’ve seen people cling to denial and call it a hoax, and we’ve seen people either carefully and rationally balancing going out or just being selfish and going out.

Governments have conducted wargame-style simulations, such as Operation Dark Winter and Exercise Cygnus, but the participants in these tend to be educated and experienced adults (and they know they are in a wargame). Participants in Frederic Chin’s 2013 online game likewise knew it was intended to look at risk behavior, but he worked to account for that by adjusting incentives.

One thing that very starkly stood out is that the disease in my game didn’t have to be a problem,” Chen says. “Everybody collectively could have eradicated the disease if they acted safe enough.” That never happened – and in fact, people performed worse than Chen had expected, so much so that he had to give some of his grant money back because he paid out far less in prizes than he had predicted.

Economists call these externalities – actions we take which have a negative impact on a third party but don’t impact the people making the decision. They’re why climate change is such a big problem, and why the environmental impact of palm oil isn’t reflected in the price of Nutella. (This para from Wired)

This is why the emergent behavior of an unexpected disease outbreak was so valuable, despite the cost being only virtual life not actual life.

“It has resulted in a pretty heavy emphasis, at least in my own research, about considering and accounting for host behavior and how people react to outbreaks, rather than assuming they’re perfectly well-behaved rational actors,” says Lofgren. In his research on preventing infections in hospitals, for example, his model now includes nurses and doctors who forget to wash their hands. And when it comes to designing better prevention for these infections, Lofgren has begun to emphasize measures that don’t rely on human behavior, which can never be fully purged of unreliability and irrationality. So instead of depending on nurses to wash their hands, it might be more effective to change the way the hospital is designed and what materials it uses.

“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.”

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.”


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