Okay, hear me out on this very basic idea. Sometime in the future humans created "pseudosaurs" by modifying bird and reptile DNA. However, some rogue nation or terrorist faction modified a "[url=https://en.wikipedia.org/wiki/Ophiocordyceps_unilateralis]zombie fungus[/url]". By a stroke of luck, these pseudosaurs are immune, or extremely resistant, to the fungus. Mankind is devestated, the majority turned into zombies trying to spread their spores.


Extremely basic, but I'll hone it.