The event involved the game character Hakkar, who just happens to be the god of blood. Anyone fighting Hakkar catches a plague, which is also highly-contagious to sub-Level 50 characters. It is thought the disease was made contagious to allow it to spread to nearby allies. No one--not even Blizzard--expected that a player would make it back to a populated area, though. Well, one did, and the character infected a whole city; the devastation went from there. The disease isn't limited to players either, as NPCs are also keeling over and dying. Quarantine methods are failing, since players carrying the disease are still free to move around the world as they please.
So, let me guess who that one player was who infected everyone else... Could it be Leroy JENKINS??