Once upon a time, Equestria was a safe place. But that was before they appeared. They aren't zombies, because they are alive. They are alive, but that doesn't mean they are rational. They aren't rational, but that doesn't mean they aren't intelligent. They are many. They are fast. They are strong. They are contagious. One bite, just a little one, and you have one hour to decide if you want to die as a pony or live as one of them.