Heres the skinny: Blizzard adds in a new instance, Zul'Gurub. Inside is the god of blood, Hakkar. Well, when you fight him he has a debuff called Corrputed Blood. It does like 250-350 damage to palyers and affects nearby players. 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 genral population including npcs, who can out generate 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 die).lolz.It was clearly unintended to ever reach outside of the instance due to the short duration. And quite likely took a significant degree of effort to spread there. In addition to the inital effort a small number of people invested significant time and effort in keeping the plague alive. They made use of non-combat pets (which cannnot die) to keep a strain alive when other players left range. They made use of healers to sustain an individual who could carry the disease around the city in high traffic areas, particular the bank, auction house, and the griffin.