20 years ago: The first pandemic in an online game

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Since September 13, 2005, strange and sinister things have been happening in Azeroth, the world of "World of Warcraft": players have been dying in the otherwise safe capital cities such as Ironforge and Orgrimmar. They are exposed to a debuff called "Corrupted Blood", which drains 200 health points from them every two seconds. This is called a debuff, the opposite of a buff. Those who have already reached level 60 will survive it, but lower levels will lose their lives.

The evil spell only lasts ten seconds, but it spreads like a virus to other players in the vicinity – and even to computer characters, the NPCs who can be found in towns as traders, teachers and quest givers. Even more disastrous is that a player can be infected again and again, as there is no immunity after contracting the disease. Death is not permanent, but since you usually revive where you died ... you are immediately infected again. It is estimated that 4 million players are affected by the blood plague.

The hunters are to blame

How can this happen? In addition to the open world, online role-playing games have dungeons, usually underground areas. They are populated by particularly strong computer opponents who leave behind particularly good rewards. Dungeons are conquered in groups, the raids. Each raid receives its own copy of the adventure, an instance to keep to itself. On September 13, 2005, another raid will be unlocked, the first for 20 players, after previous raids were intended for 40 players. It is the first raid in the open air, in the exotic jungle city of Zul'Gurub in the Stranglethorn Valley. At the end, Hakkar the Soulflayer awaits, spraying individual players with "Corrupted Blood", which deals additional damage over time in addition to an initial hit. Dot is what players call this: damage over time. The spell is transferred to nearby players –, which is why players keep their distance from each other in battle.

After the final boss, players leave the raid, usually by teleporting to a capital city. The developers are prudent enough to remove the debuff when leaving the instance so that the plague does not spread into the open world. What they don't think about, however, are the pets of hunters and warlocks, who largely fight along automatically. They also receive "Corrupted Blood" – and take it with them out of the instance, where they pass the infection on to other players.

Many players pocket their loyal companion to save it. However, this is where a second negligence for the developers comes into play: the blood spell does not disappear, but is merely frozen. If a hunter unpacks his wolf again later, the disease timer restarts and the animal infects nearby players.

NPCs as asymptomatic carriers

This could remain controllable, but the many NPCs are also infected. The debuff does not end for them, and they do not die ... because they are computer characters. Different rules and different scripts apply to them than to normal players: there is no provision for them to die, and there is no provision for a debuff to expire.

This has catastrophic consequences. If you walk past an NPC, you get infected, which is deadly for low levels. The streets are littered with the skeletons of dead players. Spooky.

Players are dealing with the pandemic in a variety of ways. Some warn, others help, some flee to remote areas. Some are careless, others deliberately infect players.

Developer Blizzard is struggling to get the pandemic under control. As with a real disease, there are two things at stake: the causes and the effects. Both are complex, especially with an ongoing online game and data from millions of characters. It takes a while to identify the causes. Several hotfixes and quarantine zones do not have the desired effect. Only after four weeks does a patch appear that prevents companion animals from contracting the blood plague. This was combined with a "hard reset", mentioned in many sources but not explained in detail, which eradicated the disease from all players.