We are talking about the Romero zombie movies, right? There are some posts I haven't sifted through in this thread, so I'm not sure. There are so many spinoffs, unofficial sequels, parodies, movies with the same name, etc. It's hard to tell sometimes...
Anyway, in the first movie (Night of the Living Dead), there's a lot of business with people on the television speculating that these creatures have something to do with a strange meteorite that appeared around the same time.
In Dawn of the Dead, there is some speculation about viruses and other stuff. It is noted that the creatures are not cannibals because they do not eat each other and only prey on regular humans. There's also the "When hell is full" line.
In Day of the Dead, it is well-established that a virus is a causal agent. No mention is made of the meteorite or if that connection turned out to be even partially right. It is known that if a limb is bitten and subsequently severed within a short period of time, the infection cannot spread to the rest of the body, but otherwise there is no treatment once a bite occurs. This is, from what I can recall, the most interesting movie of the series.
I haven't seen Land of the Dead, so I don't know if any new insights are presented there.
Is it from a mutation? Well, there's no mention of that, so probably not.
How did the little girl get it? I forget the details of that scene. However, in Night of the Living Dead, there are some characters that seem to have become zombies without having been bitten or anything. It seems to happen to anyone who dies of any causes. In the later movies, the zombies have been a presence for years, so people who die tend to die from attacks and were bitten anyway. If it's still possible to contract the infection or whatever without having been bitten, the girl may have gotten it that way. I have a friend who has watched this movie several times (and he's also a huge fan of Evil Dead 2, which I noticed Orgg recommending) and may be able to answer this question. I'll ask him if I remember...
Edit: My friend pointed out that this is referring to the remake, which he has also seen, although I have not (some people watch too many cheesy movies...). Anyway, he said the idea that people could die of natural causes and still become zombies didn't really continue into the rest of the series. As for the girl (who wasn't in the 70's movie I was thinking of), he thought her parents probably infected her and then she wandered off. Not sure if that makes sense or not...
Did they survive? Well, for how long? In Day of the Dead (not really a spoiler unless you don't want to know anything about the movie whatsoever), the zombies have already won and the story takes place in a giant underground military fortress deal where researchers and soldiers and such are still working even after they lost contact with the outside world. It would be highly unlikely that the surviving characters from Dawn of the Dead made it through all that, but maybe they're hiding on a remote mountaintop somewhere or something. Who knows? Edit: This applies to the original Dawn of the Dead. And I now remember that it didn't end in a montage, so the endings are probably completely different...
