I can not answer your question about what apocalyptic event could cause this, but I can say what apocalyptic event would end it.
It's time to take the gloves off on this 'clean apocalyptic' thing.
There are 7,500,000,000 humans, give or take a few, on earth. You are talking about most of them suddenly dying. Not vanishing, not disappearing, but dropping dead. Leaving dead bodies behind. Imagine a stadium with 40,000 people, all dead in their seats. They just stay there, rotting. Imagine a shopping mall, where everyone has just dropped dead. No one removes the bodies. There are 25,000,000 people in Beijing alone. All dropping dead overnight. No one collects the bodies. Now, how about seven billion dead bodies, all at once?
Let's look at what happens to just one dead body. Maggots set in. Blowflies lay eggs. Roaches take up residence and lay eggs. Worms take up residence. Rats come to dine. All of those pet cats and dogs, now without owners who provide their food, seek out sustenance. This is happening to not just one body, but seven billion bodies all at once, all rotting and being consumed. And this is just the human bodies. Think about all of the domesticated animals - cows, pigs, chickens, in industrial food factory farms? No one takes care of them, they die. By the millions, if not billions. And they rot, or become food for predator animals, insects, bacteria, and such..
I have absolutely no idea of what apocalyptic event might cause it, but I can tell you what apocalyptic event would follow. Not just the plague, not just some bacterial or viral infection, not just some biological weapon, not just some health crisis, but every disease known would run rampant, unconstrained. No medicines, no medical facilities, no doctors to alleviate it. Malaria, diphtheria, dysentery, lyme disease, Necrotizing fasciitis, legionnaires disease, all of them all at once, would become epidemic. No sanitary facilities. No sewage treatment. No garbage collection. Rotting food, bodies, feces stay where it lies. No quarantine. Cats, dogs, coyotes, rats, would prowl every street, every home, every store. Every pet, now a feral predator. Unrestrained reproduction of those not neutered. Untold billions of tons of food would go rotting in stores, houses, warehouses. Bacteria, viruses, mold would cover everything that humans inhabited. The stench would be unimaginable. The worst of the slums of Calcutta, on a global scale, but without living humans to put up even a basic defense. No 'perimeter' to contain it. Every imaginable disease carrier - flies, mosquitoes, rats, mice, bats - unrestrained and uncontrolled.
We are not talking about a 'clean' event - there can be no clean apocalyptic events (unless these billions of bodies, their pets, and warehoused fresh and uncanned packaged food just disappear).
We are not talking about any surviving humans dealing with the few humans you posit would remain, we are talking about these very unlucky humans dealing with every imaginable onslaught of predatory animals and disease.
There would be no 'Ideally people would have access to..'. There would be no ideally at all. There would be rot, decay, and disease on an unimaginable continuing multiple apocalyptic scale, not just one apocalyptic event.