Depending on what zombie "lore" you're going by, there doesn't always have to be a bite or scratch. In all of the original "...of the Dead" movies, corpses reanimate no matter what the cause of death was, and there are other examples which I can't recall right now. Usually they explain it as a virus or something like that, and I think even in the original "Night of the Living Dead" they mentioned something about radiation from Venus. It will be interesting though, to see how they will explain it in this story.