Corrupted Blood incident
The Corrupted Blood incident is a video game glitch and virtual plague that occurred on September 13, 2005 in the MMORPG World of Warcraft. The epidemic began with the introduction of the new dungeon Zul'Gurub and its end boss Hakkar who, when confronted and attacked, would cast on players a hit point draining and highly-contagious debuff spell called "Corrupted Blood."
The spell, intended to last only seconds and function only within the new area of Zul'Gurub, soon spread across the virtual world when players discovered that the use of teleportation spells could take the affliction out of its intended confines. By both accidental and purposeful intent, a pandemic ensued that quickly killed lower-level characters and annoyed higher-leveled ones, drastically changing normal gameplay, as players did what they could to avoid infection. Despite measures such as programmer-imposed quarantines, and the players' abandoning of densely populated cities (or even just not playing the game), the epidemic was only finally controlled with a combination of patches and resets of the virtual world.
The conditions and reactions of the event attracted the attention of epidemiologists for its implications of how human populations could react to a real-world epidemic. Anti-terrorism officials also took notice of the event, noting the implications of some players planning and perpetrating a virtual biological attack.
The epidemic began on September 13, 2005, when Blizzard introduced a new dungeon called Zul'Gurub into the game as part of a new update. Its end boss, Hakkar, could affect players by using a debuff called Corrupted Blood, a disease that damages players over time, this one specifically doing significant damage. The disease could be passed on between any nearby characters, and would kill characters off lower level in a few seconds while higher level characters could keep themselves alive. It would disappear as time passed or when the character died. Due to a programming error, players—either inadvertently or intentionally—spread this effect by teleporting out of Zul'Gurub and passing it on to other players.
Although the disease was intended as a "short-term, short-range annoyance", some players took advantage of the teleport glitch. Non-player characters could contract the disease, but were asymptomatic to it and could spread it to others. At least three of the game's servers were affected. The difficulty in killing Hakkar may have limited the spread of the disease. Discussion forum posters described seeing "hundreds of bodies" lying in the streets of the towns and cities. Deaths in World of Warcraft are not permanent, as characters are resurrected shortly afterward. However, dying in such a way is disadvantageous to the player's character and incurs inconvenience.
During the epidemic, normal gameplay was disrupted. Player responses varied but resembled real-world behaviors. Some characters with healing abilities volunteered their services, some lower-level characters who could not help would direct people away from infected areas, some characters would flee to uninfected areas, and some characters attempted to spread the disease to others. Players in the game reacted to the disease as if there was real risk to their well-being. Blizzard Entertainment attempted to institute a voluntary quarantine to stem the disease, but it failed, as some players didn't take it seriously, while others took advantage of the pandemonium. Despite certain security measures, players overcame them by giving the disease to summonable pets. Blizzard was forced to fix the problem by instituting hard resets of the servers and applying "quick fixes".
The major towns and cities were abandoned by the population as panic set in and players rushed to evacuate to the relative safety of the countryside, leaving urban areas "filled to the brim with corpses", and the "city streets literally white with the bones of the dead".
World of Warcraft, at the time, had more than two million players all over the world. Before Blizzard Entertainment commented on the outbreak, there was debate whether it was intentional or a glitch. On Blizzard's forums, posters were commenting about how it was a fantastic world event, and calling it "the day the plague wiped out Ironforge." It was described as the first proper "world event" by an editor of a World of Warcraft fan site. After the incident began, Blizzard received calls from angry customers complaining about how they just died. Some players abandoned the game altogether until the problem was fixed. The hard resets were described as a "blunt ending" by Gamasutra.
The people who spread the disease out of malice were described by Security Focus editor Robert Lemos as terrorists of World of Warcraft. He commented that this might be the first time a disease passed from player to player in a game. Mark Ward, an editor for the BBC's web site, brought up an incident years earlier in the computer game The Sims, where many players' characters died of a curable disease originating from a dirty guinea pig, though there was no way for the disease to spread to a different player's character.
Jeffrey Kaplan—a game designer for World of Warcraft—commented that it gave them ideas for possible real events in the future. Brian Martin—independent security consultant for World of Warcraft—commented that it presented an in-game dynamic that was not expected by players or Blizzard developers and that it reminds people that even in controlled online atmospheres, unexpected consequences can occur. He also compared it to a computer virus, stating that while it is not as serious, it also reminds people of the impact computer code can have on them, and they're not always safe, regardless of the precautions they take.
Great Zombie Plague of '08 
During one week of October 2008, a zombie plague was spread to promote the second World of Warcraft expansion, Wrath of the Lich King, before its release. Unlike Corrupted Blood, this plague was intentional and was dubbed by an authorized representative of Blizzard Entertainment as the "Great Zombie Plague of '08". It was compared to Corrupted Blood by The Sunday Times, who described the zombie plague as being more true-to-life. The plague was highly contagious, but in contrast to Corrupted Blood, which had 100% transmission to nearby characters, being in the vicinity of a character infected with the zombie plague represented only a small risk of transmission. This meant that encountering a lone zombie was not as dangerous as encountering a large mass of infected. The event—which Blizzard ended on 28 October—earned the company both praise and criticism from its fans.
Models for real-world research 
Model for epidemic research 
In March 2007, Ran D Balicer, an epidemiologist physician at the Ben-Gurion University in Israel, published an article in the journal Epidemiology describing the similarities between this outbreak and the recent SARS and avian influenza outbreaks. Dr Balicer suggested role-playing games could serve as an advanced platform for modeling the dissemination of infectious diseases. In a follow-up article in the journal Science, the game Second Life was suggested as another possible platform for these studies. The Centers for Disease Control and Prevention contacted Blizzard Entertainment and requested statistics on this event for research on epidemics, but was told that it was a glitch.
The Corrupted Blood incident was described as a fascinating, yet accidental, case study of modeling disease origins and control at the Games for Health conference in Baltimore, Maryland by Gamasutra. They compared it to a real-life epidemic, in that it originated in a remote, uninhabited region and was carried by travelers to larger regions; hosts were both human and animal, comparing it to the avian flu; was passed through close contact; and there were people, in this case non-playable characters, who could contract it but were asymptomatic. However, there were elements that differed from a real-world epidemic, including an indicator for carriers that they have the disease and how much risk they are at, which cannot be done in the real world. One aspect of the epidemic that was not considered by epidemiologists in their models was curiosity, describing how players would rush into infected areas to witness the infection and then rush out. This was paralleled to real-world behavior, specifically with how journalists would rush toward a problem to cover it, and then rush back out.
In August 2007, Nina Fefferman—a Tufts University assistant research professor of public health and family medicine—called for research on this incident, citing the resemblances with biological plagues. Some scientists want to study how people would react to environmental pathogens, by using the virtual counterpart as a point of reference. Subsequently, she co-authored a paper in Lancet Infectious Disease discussing the epidemiological and disease modeling implications of the outbreak, along with Eric Lofgren, a University of North Carolina graduate student. She spoke at the 2008 Games for Health conference in Baltimore, Maryland and the 2011 Game Developers Conference about the incident and how massively multiplayer online populations could solve the problems inherent with more traditional models of epidemics.
Fefferman added that the three base models have their strengths and weaknesses, but make significant behavioral assumptions. She also compared Corrupted Blood to a drug trial with mice—"a real good first step." She stated that "these are my mice" and that "I want this to be my new experiment setup." She expressed an interest in designing new diseases, perhaps non-fatal ones, to be introduced to the game so she could study how risk is viewed, how rumors would spread, and how public health notices are handled. She added that Blizzard made such notices in the original outbreak, but kept changing its position as it could not effectively deal with the problem. She commented that she did not believe it would ruin gameplay, as World of Warcraft dealt with health challenges in combat, and that games set in medieval times had such health risk. She argued that if researchers and developers worked together, it could be fun. While Blizzard was initially excited about the proposition, it became less outwardly excited over time, though never rejected it. She has been in contact with other developers, hoping to conduct the simulation in similar games to World of Warcraft.
She thought that this was the only way to accomplish such a study, as epidemiologists were limited to observational and retrospective studies, because it would be immoral to release an infectious disease into the population. She added that a computer model would be insufficient as well, as it uses mathematical rules to approximate human behavior. Doctor Gary Smith, professor of Population Biology and Epidemiology at the University of Pennsylvania, commented that very few mathematical models of disease transmission take host behavior into account, but also questioned how representative of real life a virtual model could be. He stated that while the characteristics of the disease could be defined beforehand, the study is just as observational as one conducted on a real-life disease outbreak. However, he added that one could argue that the proposal could give an opportunity for a study that epidemiologists may never have. Neil Ferguson, director of the MRC Centre for Outbreak Analysis and Modelling at Imperial College, London, felt skeptical of the idea, commenting that such a study could not properly mimic genuine behavior. Using the zombie plague used to promote World of Warcraft: Wrath of the Lich King before its release as an example, players would intentionally become infected to gain zombie powers. He added that characters could also regenerate, meaning there was low risk in becoming infected. He felt that while online games such as World of Warcraft could be set up to help scientists study epidemics, it will always be limited as their primary use is for entertainment.
Model for terrorism research 
In an analysis of the Corrupted Blood incident, Charles Blair, deputy director of the Center of Terrorism and Intelligence Studies, said that World of Warcraft could provide a powerful new way to study how terrorist cells form and operate. While his organization already uses computer models to study terrorists' tactics, Blair explained that because World of Warcraft involves real people making real decisions in a world with controllable bounds, which could provide a more realistic models for military intelligence analysts. For example, one self-confessed virtual bioterrorist in World of Warcraft commented about how quickly people got smart about doing the most damage to the largest number of people.
Yale University terrorism expert Stuart Gottlieb admitted that while the outbreak was interesting and relevant to the times, he would not base a counter-terrorism strategy on a video game. Gottlieb expressed skepticism that analyzing the incident could shed light on the complex underlying causes of terrorism in the real world, as the stakes for both terrorists and civilians are lowered in a virtual setting. However, as commented by the editor of the article, "the biggest weakness for using a game as an analytical tool is that death in World of Warcraft is a nuisance at most." 
Blizzard has maintained a position that World of Warcraft is first and foremost a game, and that it was never designed to mirror reality or anything in the real world.
- Orland, Kyle (2008-05-20). "GFH: The Real Life Lessons Of WoW's Corrupted Blood". Gamasutra. United States: Game Developer. Retrieved 2009-02-28.
- Ward, Mark (2005-09-22). "Deadly plague hits Warcraft world". BBC News Online (in British English) (London, England, United Kingdom: BBC). Retrieved 2009-02-28. Unknown parameter
- McNamara, Tom (2005-09-22). "World of Warcraft Gets Plagued". IGN. New York City, New York, USA: News Corporation. Retrieved 2009-03-03. "In-game disease spreads throughout the realm."
- "Virtual game is a 'disease model'". BBC News Online (in British English) (London, England, UK: BBC). 2007-08-21. Retrieved 2009-03-13. "An outbreak of a deadly disease in a virtual world can offer insights into real life epidemics, scientists suggest." Unknown parameter
- "World of Warcraft Shines Light on Terror Tactics". Wired.com. 2008-03-20. Retrieved 2009-02-27.
- "Online 'blood plague' offers lessons for pandemics". Reuters. 2009-04-27.
- "World of Warcraft plague 'swamps servers'". The Register. 2005-09-21. Retrieved 2009-02-27.
- "Looking Back... World of Warcraft". Computer and Video Games. 2005-01-04. Retrieved 2009-02-26.
- "Virtual worlds, real science: epidemiologists, social scientists flock to online world.(plague in the World of Warcraft)". Science News. 2007-10-27. Archived from the original on 2009-03-15. Retrieved 2009-02-27.
- "Digital plague hits online game World of Warcraft". Security Focus. 2005-09-27. Retrieved 2009-02-26.
- Tigole (2008-10-27). "World of Warcraft - English (NA) Forums -> Regarding the Zombie Plague…". Blizzard Entertainment. Archived from the original on 2010-10-19. Retrieved 2010-10-19.
- Henderson, Mark (2008-10-28). "Analysis: Absence of risk limits parallels with real life". The Sunday Times (in British English) (United Kingdom: News International). ISSN 0956-1382. Archived from the original on 2010-10-19. Retrieved 2010-10-19. "Why avoid anything like the plague if you can just come back to life?"
- Ahmed, Murad (2008-10-28). "World of Warcraft plague halted by Blizzard". The Sunday Times (in British English) (United Kingdom: News International). ISSN 0956-1382. Archived from the original on 2010-10-19. Retrieved 2010-10-19. "The Warcraft's authorities have acted to control a virtual plague they released into the popular online game last week"
- Balicer, Ran (2005-10-05). "Modeling Infectious Diseases Dissemination Through Online Role-Playing Games". Epidemiology 18 (2): 260–261. doi:10.1097/01.ede.0000254692.80550.60. PMID 17301707.
- This Second Life avatar looks healthy--so far 316 (5827). 2005-10-05. p. 961. doi:10.1126/science.316.5827.961a.
- Sydell, Laura (2005-10-05). "'Virtual' Virus Sheds Light on Real-World Behavior" (Streaming audio). NPR. Retrieved 2006-08-05.
- Bannerman, Lucy (2007-08-21). "How a computer game glitch could help to fight off global pandemic". The Times (London). Retrieved 2007-08-21.
- Lofgren, E.T. and N.H. Fefferman. 2007. The Untapped Potential of Virtual Game Worlds to Shed Light on Real World Epidemics. The Lancet Infectious Diseases. 7:625-629 PMID 17714675 doi:10.1016/S1473-3099(07)70212-8
- Graft, Kris (2011-02-28). "GDC 2011: An Epidemiologist's View Of World Of Warcraft's Corrupted Blood Plague". Gamasutra. Retrieved 2011-03-16.