There's this thing called rain, where fresh water fall from the sky. I know I sound like a crack head but its true!!!the first thing you do (aside from grabbing your sawed off shotgun and miramasas) is fill up your bathtubs with water (you can figure out how to clean the water later) -- this is because you don't know when water will become a rarity.
Unless you get zombie factories and industries making acid rain. But if injuries prevent the operation of heavy machinery I think death would also be the same situation