The first sign of trouble was the arrival of the Argent Dawn, who appeared in capital cities throughout Azeroth. Said the Dawn, "The plagued tendrils of the Lich King lurk close." Scary!
Then, the zombies.
The walking dead appeared outside major cities, and players unlucky (or lucky?) enough to come in contact with them were themselves infected with the zombie plague. Players have 10 minutes to find a healer before the plague takes hold. Becoming a zombie isn't as bad as it sound, though. Players are granted a new status bar and zombie abilities, which last until the player is cured by a healer.
And then the zombie crates showed up.
In the Booty Bay area, "Conspicuous Crates" appeared last night that can also infect players. The results, after 10 minutes, are the same as the zombies.