I have heard (not confirmed) that the disease was accidentally made like Azuregos' Mark of Ice, and it is also (on purpose) like Trogg's Radiation --- meaning that a) it affects not only living players, but their corpses and ghosts, and b) that it transfers from one player to another by an event that triggers whenever you take damage.
This would mean that it would affect players, their corpses, their pets, their pets' corpses, their ghosts, and their non-com pets. The problem is that, then, any player walking next to an infected corpse... it's kind of like the plague.
The way it devestated Archimonde was simple. A hunter, after killing hakkar, dismissed his disease-ridden pet. A few minutes later, in Ogrimmnar, he resummoned it in the Auction House. Suddenly, a very powerful disease began to infect the AH. Players, engrosed in their shopping, would often not notice the disease at all until they were standing in the graveyard. Because the disease is rumored to transcend death, the very act of walking to retrieve your corpse would cause living players near your ghost to suddenly get sick.
Widespread hordie death. And apparently on Archimonde they had a transference over to the Alliance too, in STV. FUN!!!
lol well it was an amusing bug.