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.

Saw this article before, still think its genious developing...

It is similar to the "Soulbomb" problem from a few months ago.
Its a DoT that spreads, so if someone gets it, then heads back to civilization quickly, they can spread it.

In MUD2, players stand a chance of catching a cold if they sleep outside in the open and it starts to rain. When they have a cold, there's no cure (no magic is that powerful) and they'll occasionally sneeze and cough, reducing their stamina (hit points). If you sneeze or cough in the presence of another humanoid, they could catch your cold too (unless they only recently had it, in which case they're immune).

Most colds were spread between players, but occasionally a humanoid mobile would get hurt in a fight, sleep (to recover stamina) in the open, and catch a cold. This mobile would then cough on players and perhaps infect them.

Naturally, players took (well, take, as they still do this) this cold and use it as a weapon. The usual tactic is to catch a cold, visit a realm inhabited by some 52 (NPC) dwarfs, and infect one of those with it. They'll pass it to other dwarfs, until eventually most of the dwarf population has a cold. Dwarfs have high strength and dexterity, so they're tough in fights, but their stamina isn't so high; losing 3 stamina to a cough is therefore more worrisome for a dwarf than for a player character. Players will let the infection spread, then weigh in and attack the dwarfs that have reduced stamina or, better still, that are sleeping (no defence against an attack!) to recover the stamina they just coughed away.