In the dungeons of Zul'Gurub frequented by online game enthusiasts, a giant winged serpent called Hakkar the Soulflayer may offer important clues to epidemiologists trying to predict the impact of a pandemic.

In September 2005, a plague called Corrupted Blood caused mayhem in the hugely popular online game World of Warcraft. What happened next illustrates the kind of issues policymakers will have to grapple with if a deadly outbreak of swine flu in Mexico spreads.

An estimated 4 million players were affected by the pandemic, and by the time it had run its course, whole virtual cities were littered with the bones of the dead, with most survivors fleeing urban areas for the relative sanctuary of the countryside.

Epidemiologists and disaster planners have tried for years to build realistic models of how a highly virulent disease might spread and impact global society and the economy.

But the Corrupted Blood plague accidentally provided something unprecedented -- a chance to safely study a pandemic in a uniquely complex virtual environment in which millions of unpredictable individuals were making their own decisions.

In an article in the Lancet Infectious Diseases journal in 2007, Nina Fefferman and Eric Lofgren of the Tufts University School of Medicine said the incident raised the possibility for valuable scientific content to be gained from this unintentional game error -- providing insight into real-world pandemics.

Blizzard Entertainment, the creators of World of Warcraft, never intended the plague to get so out of control.

At first, it could only be encountered by relatively advanced players who had penetrated deep into a new dungeon provided as part of a software update. Among the many offensive powers of Hakkar -- others included blood siphon and cause insanity -- was the ability to spread the Corrupted Blood plague.

Most players who had come this far were strong enough to survive, and the plague was never supposed to spread far. But as with many real-world events, things did not go entirely to plan.

TELEPORTATION AND MAGIC PETS

Unlike previous 'virtual plagues' that had been officially planned, this was a local effect that went out of control -- a naturally occurring virtual outbreak, Ran Balicer of Israel's Ben-Gurion University wrote in the journal Epidemiology.

World of Warcraft allows players to teleport themselves from one place to another. And some infected players beamed themselves out of the dungeon and into cities, where weaker victims soon began to drop like flies. Also, the game allows players to own pets who can be banished and summoned at will. These pets also played a key role in spreading the plague beyond the dungeon.

While teleportation and magical pets belong in the world of fantasy, these developments inadvertently mimicked possible characteristics of real pandemics, the two studies say.

The risks that pandemics will spread through quick global transportation links and inter-species transmission are key factors that may well come into play in real-world outbreaks.

Balicer said the impact of teleportation in World of Warcraft was similar to the role of air travel in the rapid global spread of severe acute respiratory syndrome (SARS), while the havoc wrought by infected pets echoed the part played by asymptomatic ducks in spreading avian flu among bird populations.

Before long, disease was raging across the online world.

Soon, the disease had spread to the densely populated capital cities... causing high rates of mortality and, much more importantly, the social chaos that comes from a large-scale outbreak of deadly disease, Fefferman and Lofgren wrote.

In web forums, players described scenes of devastation. One said the online world had been filled to the brim with corpses.

The city had streets literally white with the bones of the dead, the player said. Screenshots show game characters walking through eerily deserted streets strewn with skeletons.

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... to become the first online instance of uncontrolled plague to affect millions of Americans, Asians and Europeans at home, Fefferman and Lofgren wrote.

HUMAN RESPONSES

What made Corrupted Blood so interesting was the way players responded -- providing an insight into the psychological response to plague that most computer models can never hope to capture.

Some players selflessly rushed to help, using their healing powers and acting as first responders despite the risk.

Their behavior may have actually extended the course of the epidemic and altered its dynamics... 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, the Lancet article said.

Others got infected on purpose and strolled around populated areas -- leading some security analysts to say the incident may provide insight into how terrorists would exploit a pandemic.

Dealing with thousands of complaints from players whose online alter egos had fallen victim to the plague, Blizzard Entertainment tried to quarantine the infected zones. But -- as is likely in real life -- the barriers were porous and some infected victims managed to find their way into safe areas.

The inability of the game's creators to halt the virtual plague mirrored expectations of real epidemics.

Once a fully contagious virus emerges, its global spread is considered inevitable, the International Monetary Fund said in a 2006 report on the likely impact of a flu pandemic. Countries might, through measures such as border closures and travel restrictions, delay arrival of the virus, but cannot stop it.

Confusion reigned in Internet chat forums as baffled players tried to understand the plague -- often spreading misinformation.

Again, this has real-world parallels. In a study of the likely effects of a major pandemic, the World Economic Forum warned of the risks of infodemics -- where the rapid spread of inaccurate or incomplete information can amplify the effects of the core risk event.

In the end, Blizzard had to cheat to save the world.

The game's developers (had) an option that remains unavailable to public health officials -- resetting the computers, Fefferman and Lofgren wrote.

When the servers ravaged by the epidemic were reset and the effect removed, the outbreak came to a halt.

(Editing by Mark Trevelyan)

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