We talk about the ZA being a virus that reanimates some parts of the brain, or a curse from above, or an evil plague, etc.
How about this: a bunch of aliens looking for a fresh planet, no firepower to go head-to-head against a nuke-armed aggressive human race, but they need our world. They have the biotechnology to introduce this virus, and slow down rotting, and they just need to seed our atmosphere with the virus and sit back for a few years, then come in and take over.
Maybe they are really fragile, maybe they are a long-lived species and therefore totally risk-averse...whatever it is, they want humans to kill each other off before they show up.
THAT would be quite the epic story! Enders Game meet Starship Invaders meet
George Romero Rick Grimes...
Season 19 we'll see Michonne with a light sabre, and Daryl with his liquid-metal plasma bolt projector.
But anyways, if this was a bunch of cowardly aliens, what is the minimum level of government forces in existence to deter them? How would the new world look like, and how would human survivors manage, with the low-level Z threat and the high-level alien threat?