When it comes to Post-Apocalyptic stories, the nay-sayers are pretty quick to point out how unrealistic a scenario is. There’s always some factor or instance that, they think, would undercut the apocalypse and forestall subsequent doom. The problem with this view is that the Post-Apocalyptic genre isn’t stories of the world almost ending, it’s blatantly about scenarios where the world hasn’t ended.
It’s far more likely that the world doesn’t end in some apocalyptic event, that’s a given. If we woke up every day and had to deal with the dread of some random apocalypse transpiring, we’d never get anything done. We wouldn’t have even moved out of our caves, why would we have bothered? The point is that 99.9999% of the time the world will not end, but Post-Apocalyptic stories are about that “perfect storm” scenario where it does. The catastrophic clusterfuck of an apocalypse is found in the insanely infinitesimal.
Nobody cares about an averted zombie apocalypse. One guy gets turned undead in a lab experiment, breaks free and then infects the scientist, the zombies run rampant through the top secret military complex but they’re eventually taken out by special forces, or a nuke… it doesn’t matter. A recently killed woman rises from her grave and hunts down her murderer, but a local deputy sees her shambling corpse and blows her head off then drags her back to her grave and buries her again. The Post-Apocalyptic genre isn’t about how the world is almost destroyed, they’re your simple Action/Thriller/Drama/Horror stories. The Post-Apocalyptic genre is about the cascading failure that does, despite all odds, result in absolute destruction for humanity.
Sure, there’s enough security around highly virulent diseases that the chances of one of them breaking free and causing a pandemic are slim, but this is about the time that the precautions in place juuuust weren’t enough. There’s a crash while the vials are in transport, the vehicle’s containment cases weren’t locked properly, the virus gets out. A fire causes a black out and the old generators fail to kick in, all the doors unlock due to the fire, the virus gets out. A scientist caught his brother sleeping with his wife, he goes to work and thinks “fuck the world” and… the virus gets out.
A lot of small things need to happen for an entire world to collapse, and total ruin could potentially be averted if any of them fail to happen. Go Google those times that the world was nearly drowned in nuclear hellfire because of a flock of birds, or because of some sunlight refracting off of clouds. As difficult as it is for an apocalypse to eventuate, in spite of all the precautions, it’s often in the cracks of absurdity that it slips through.
It’s not about how unrealistic the scenario is, as long as the author has put in the work to make it as realistic as possible, then it’s as realistic as it needs to be. If you’re working on a Post-Apocalyptic story, don’t think of it as destroying a functioning world but instead think of it as creating a destroyed world. Think of how it could happen, don’t get bogged down in all the ways it couldn’t. Sometimes simple bad luck can ruin your life, and it can do the same to the world.