No one quite knew where the virus came from. Certainly, someone put the cas9 gene and guide RNA sequences into the Feline Herpesvirus vector. Someone carefully selected the genes to be mutated: genes involved with the ability to form articulate speech, the size of the pre-frontal cortex, and the development of opposable thumbs and bipedal posture. Whether they'd engineered the virus to be so promiscuous across species or if that mutation occurred at random, possibly accelerated by the nuclear fallout, was another question. Either because the virus needed higher temperatures to replicate or because there wasn't enough of the right genetic material to work with, ectothermic vertebrates like reptiles and amphibians seemed to be immune. Some people said that at least two strains of the virus must have been engineered, one for mammals and one for birds. However, by the time people noticed the assemblages of bipedal cats seemingly conversing amongst themselves, there were bigger things to worry about: the radioactive fallout from WWIII; the escaped biological weapons that could necrotize your liver, brain, or heart in a matter of hours; and the sudden volcanic activity in the mid-Atlantic belching ash into the sky. Christians called it Armageddon, but if it was, Jesus must have thought better of getting involved.
    Some humans managed to survive in subterranean caverns with artificial light powered by geothermal energy to grow enough food to scrape by. Those who remained on the surface succumbed to disease or the suddenly wily and vicious packs of dogs. Sealed away in their caverns while they waited for the radioactivity to decline and the weapon-grade diseases to burn themselves out, these pockets of humans scraped by, trying to preserve what little of their civilizations they had left. For a hundred generations or more, they waited, but when they finally emerged, the world they found was not the ruins engulfed by untamed wilderness they'd expected.
    What these bewildered humans found instead was thriving civilization! Had humans on the surface survived?! However, what they found was far more strange: animals (mammalian for the most part) living in houses and farming fields. They even seemed to have some technology: electricity for heating and cooling powered by solar, wind, and geothermal power; trains for transportation; and some machinery for farming. Despite this, they seemed somewhat less industrialized than might have been expected with small villages surrounded by enormous forests and a long walk along dirt roads to the nearest train station. The humans soon learned the cause for this: the villages they observed held only a small fraction of the population, specifically mammals of class Carnivora. The forests were home to the raptors and, more importantly, all of the “prey" species that the carnivores and raptors ate!
    Not knowing this, the humans, of course, initially approached the villages, the most obvious sign of civilization. The felids, canids, and mustelids who greeted them were curious and hospitable. That was until someone remarked on the humans' incisors. The second human scouting parties were more cautious.
    The humans soon realized that most “prey" species, such as they were now considered, also lived underground and only emerged to forage and work their gardens. However, most humans, especially the religious “image of God" fanatics, were unwilling to accept this status. They attacked the unarmed carnivores with whatever weapons they could fashion, but they found out the hard way that firearms were not some knowledge lost to the ages but rather carefully guarded to prevent them from falling into prey's hands. Many humans were killed, and those who betrayed their comrades by leading the predators back to their caverns found their entire civilizations taken prisoner and ultimately devoured.
    The humans who survived took to living on the outskirts of the new civilization. However, because they would kill predators and prey alike and killed out of fear as much as necessity, they became a reviled species that no other species trusted. The final nail in the coffin for their relationship with the nonhuman peoples was when the animals realized they were not some new species of giant, hairless rodent but the monsters who had enslaved their ancestors and almost destroyed the world multiple times over with their violence and greed! It turned out that the animals hadn't merely been able to recreate human technologies like LEDs and microwaves but had learned to read humans' writing! And while many works of literature humans had written had been adapted for their new readers and were quite popular, the records of humans' atrocities far overshadowed any goodwill the animals might have felt toward the descendants of their favorite authors!
    In the centuries since humans reemerged onto the surface of the Earth, predation has been illegalized, and predators and prey have learned to live in peace. However, the relationship of humans with the nonhuman peoples has never recovered, and those humans who didn't return to their caverns, maybe to reemerge after another hundred generations, have become involved in the darkest parts of the underworld of the new societies. Trafficking organs and slaves they remain the most reviled of the civilized species.