This idea is cooler than some think. They announce a very vague report, "There is some type of disease spreading through the wastelands. All people be safe on (insert certain date)!" Then on the said day the server becomes a giant match of living dead. Every person has to watch out. The city walls won't be breached until a certain number of players on the outside have been turned, but there will be a way higher loot chance when people are out hunting while the living dead are running around. It would give the fear of panic, "Man o' man, we better hurry with this cabal kill run, I'm getting this feeling that the geeks are right around the corner. If you pull it off the cabal run will have a way higher chance to give you sweet, sweet gear. If you get caught then, crap there is no loss but the fact that you have to run around and get people.