at first it's just regular zombies, but before people turn, they get really jumpy and easy to aggrevate over tiny things. And at one point, it's shown how the zombies in the city has formed tribes, purely based on locations. sometimes they'll come out and fight one another to eat. one of the non-infected characters remark "well, i guess when there's not enough living people to fight over, the zombies will eat their own"
