This is from GoNintendo...|
As explained by Ubisoft, in the game, if you get bit by a zombie, your character will turn into a zombie. When you continue, you come back as a different person and the old-you is now a zombie in the game.
In the CG trailer, you see people being attacked by zombies. What's actually happening is each person you see being attacked has already been bitten. If you look closely, you can see evidence of the infection on his or her skin. The person being attacked then becomes a zombie...that is, the zombie you see in the next clip! And this cycle continues until the very end. Then it resets because the last guy in the red coat (who's chopping off a zombie head) is the zombie being thrown out the window in the very first scene."