The Corrupted Blood plague of 2005 was so virulent that, even without willing player carriers, it would have spread throughout Azeroth perfectly well on its own. Last night's plague epidemic was quite different: it had no ability to spread on its own at all. The infection came entirely from the hands of death knights themselves who, true to their nature, decided global extinction was the path to success when presented with the ability to enact it.
Two days ago, September 13, marked 7 years since the original Corrupted Blood incident. A new epidemic is certainly one way to celebrate the anniversary, but WoW Insider recommends a candlelight vigil as a potential alternative for 2013.
Gallery: Death Knight plague epidemic