These are all books about the end of the world and what happens to any survivors, from things like Natural disasters to Man Made disasters - perhaps we are heading for a combination of both!
Yes at the end of the world we all need a companion, the cats have probably all fended for themselves, so it's just you and the dog.
Some are already here - at least I hope so!
Some of these could be a parallel universe or an alternate reality
People are cruel to Zombies, they're just hungry poor things.