Here's the short version:
Blizzard adds in a new instance to their MMORPG World of Warcraft. This instance is named Zul'Gurub. Inside is the god of blood, Hakkar. When you fight him he has a debuff called Corrputed Blood. It does roughly 250-350 damage to your character and affects nearby player characters.
The amazing thing is some players have brought this disease (and it is a disease) back to the towns, outside of the instance. It starts spreading amongst the general population including NPCs, who can regenerate fast enough to ignore the damage.
Some servers have gotten so bad that you can't go into the major cities without getting the plague (and anyone less than like level 50 nearly immediately dies). GM's even tried quarantining players in certain areas, but the players kept escaping the quarantine and infecting other players.
read about it here...
When you stop to think about it, this is pretty incredible.