Since its launch in November 2004, World of Warcraft (Wow) has become the most widely played massively multiplayer online (MMO) game in the world.

Its creator, Blizzard, claims that now more than four million people are regular players.

Last rites

Wow is an online game that gives players the chance to adventure in the fantasy world of Azeroth that is populated by the usual mixture of humans, elves, orcs and other fantastic beasts.

As players explore the world, the characters they control become more powerful as they complete quests, kill monsters and find magical items and artefacts that boost abilities.

In the last week, it added the Zul'Gurub dungeon which gave players a chance to confront and kill the fearsome Hakkar - the god of Blood.

In his death throes Hakkar hits foes with a "corrupted blood" infection that can instantly kill weaker characters.

The infection was only supposed to affect those in the immediate vicinity of Hakkar's corpse but some players found a way to transfer it to other areas of the game by infecting an in-game virtual pet with it.

This pet was then unleashed in the orc capital city of Ogrimmar and proved hugely effective as the Corrupted Blood plague spread from player to player.

Although computer controlled characters did not contract the plague, they are said to have acted as "carriers" and infected player-controlled characters they encountered.

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Body count

The first server, or "realm" as Blizzard calls them, affected by the plague was Archimonde; but it is known to have spread to at least two others.

The spread of the disease could have been limited by the fact that Hakkar is difficult to kill, so some realms may not yet have got round to killing him and unleashing his parting shot.

The digital disease instantly killed lower level characters and did not take much longer to kill even powerful characters.

Many online discussion sites were buzzing with reports from the disaster zones with some describing seeing "hundreds" of bodies lying in the virtual streets of the online towns and cities.

"The debate amongst players now is if it really was intentional although due to the effects of the problem it seems unlikely," Paul Younger, an editor on the unofficial worldofwar.net site, told the BBC News website.

"It's giving players something to talk about and could possibly be considered the first proper 'world event'", he said.

Luckily the death of a character in World of Warcraft is not final so all those killed were soon resurrected.

Blizzard tried to control the plague by staging rolling re-starts of all the servers supporting the Warcraft realms and applying quick fixes.

However, there are reports that this has not solved all the problems and that isolated pockets of plague are breaking out again.

The "Corrupted Blood" plague is not the first virtual disease to break out in online worlds. In May 2000 many players of The Sims were outraged when their game characters died because of an infection contracted from a dirty virtual guinea pig.