Blizzard has just posted some more lore regarding the Undead Plague that brought the Scourge to Azeroth.

Through the course of his experimentations, Ner'zhul insured that the affliction would specifically target humans for “undeath”. Though non-human races and creatures (and even the land itself) were susceptible to the plague, it was humanity in particular that Ner'zhul meant to scour from the world. As a result, infected flora and fauna reacted differently--diseased and decaying, but not truly undead, and not under the thrall of the Lich King.

Read the rest of the lore here!