"The trouble started when Blizzard programmers added a new instance, which is a separate area connected to the outside world that players can enter and attempt unique quests. One of these instances, Zul'Grub, contained the god of blood, Hakkar. Hakkar was a powerful foe that could cast spells of his own, including a spell called Corrupted Blood. This spell did a large amount of damage to any player within the vicinity of the casting, and the effects lingered on after the spell was over.

What happened next was something Blizzard did not expect. Some of the players who had gone into the instance emerged back into the main world of Azeroth, and started spreading the Corrupted Blood disease to others who they came into close contact with. The infection soon spread into many of the cities and towns in the virtual world. Since the disease was intended to be a danger to powerful players, it tended to kill those less than level 50 almost instantly.

Game masters (GMs) tried to quarantine certain players from moving into new areas, but they kept escaping the quarantine and moving on to infect other people. A patch was issued to try and mitigate the damage, but it did not have the desired effect."

http://arstechnica.com/news.ars/post/20050921-5337.html

• I must admit that I have no idea how they are unable to control this problem, it seems that they could just remove the "disease" from all users, but it's interesting nevertheless. This is a game right?