I firmly believe that William Gibson nailed it with the Jackpot in his recent books:

nothing you could really call a nuclear war. Just everything else, tangled in the changing climate: droughts, water shortages, crop failures, honeybees gone like they almost were now, collapse of other keystone species, every last alpha predator gone, antibiotics doing even less than they already did, diseases that were never quite the one big pandemic but big enough to be historic events in themselves.