It's fair to note it as the most probable of the "Zombie Apocalypse" category, but "impossible" isn't the common theme of that category, it's "civilization endi

It's fair to note it as the most probable of the "Zombie Apocalypse" category, but "impossible" isn't the common theme of that category, it's "civilization ending". Under all of the scenarios in Problem Space #3, the world is irrevocably changed and there is not necessarily any reason to think it will go back to the way thing are now in the survivors' lifetimes. In the other two problem spaces, you are looking at perhaps-widespread but finite disruptions. Maybe an entire country or region of a large country is hosed for years or decades, but you can either expect things to go back to normal eventually, or you can migrate to somewhere where things are normal. With either a zombie apocalypse or a nuclear one, you don't have that expectation, and whatever you do to prepare for that situation will be largely the same.