It came from out of nowhere, a virus that mutates animals and humans in to anthropomorphic form, causing most humans to go insane and turn feral. The virus spread faster than any other, it took all of three days to infect almost the entire world, only few survivors remained in this furr-pocalypes, how this plays out is up to them.
Finer details can be brainstormed.
Finer details can be brainstormed.