The regular humans are locked in their houses and they are keeping quiet, so they don't attract the zombies. Once the regular humans are killed, they don't return as human, but as zombies.

If we make it so that the zombies attack the humans, then all you need to do is open all the doors and let the zombies take care of the rest! That might be too easy, don't you think?

Than again, there won't really be many zombies until the team finds some in searches or creates some, so if that's how players want to play it, I'm fine with that too.