Emergence kills, kids!
Clever/fiendish/unsuspecting World of Warcraft players have introduced a virulent in-game plague to population centers. A vicious damage over time debuff, Corrupted Blood spreads through proximity to friendlies.
It has a short lifespan and exists chiefly in one high-level dungeon, but the aforementioned cleverboots/fiends/unsuspectoids brought it back to town, for example, by recruiting healers to keep Typhoid Marys standing. They targeted, among other places, cities home to new players.
The sagas will tell of skeletons knee-deep in the streets.
The plague wasn't intended to leave the instance where it was found, but as we've seen before, players are fond of telling stories the game designers didn't intend.
What's amazing to me is that the disease has vectors like real-world ones: some exotic ailment is contracted out in the wild then exposed to cities, where it wipes out some vulnerable segment of the population before dying out. It even has animal carriers, non-combat pets. Plague pandas!
Of course, evil rage-virus labs and occasional sociopaths aside, plagues aren't spread deliberately. Griefers went after newbies but also auction house patrons and gold farmers, bringing a economic slant to the events.
Corrupted Blood hasn't broken wide on many servers yet -- and may not at all if the developers quash it -- but as news spreads you can be sure the cooties will as well. If there's anything more virulent than a plague, it's news of an exploit.