Basically the idea goes like this..
All players start on the same team, say red, and are "human", excapt one player who start on different team, say blue, and is the "zombie". The goal of the zombie is to kill all the humans. all the humans that are killed become zombies.
If the Human hit the zombie, the zombie is paralyzed for say.. 10 seconds. so that the human can run away.. The last person to not be a zombie wins.
sounds cool?