It's similar to the thing the did about 2.5 years ago.
Patch 1.1something when Naxxramas opened and there was a Scourge invasion in the cities.
The same deal is happening now in the ramp up to the Wrath of the Lich King expansion, but with the added effect of players being turned into zombies. They have full control of themselves but are usually torn up by NPC guards or other players.
What's happening now is this plague has (purposely) progressed so rapidly that you have a minute to get rid of the infection or you become a zombie, and it's rather easy to get infected. Players are also infecting NPC's such as the bankers and auctioneers removing them from the game for a few minutes until they repop. People are screaming bloody murder. I think it's great.