In 1949, a mysterious virus was found on an island and taken back to the mainland for further study. However, at least 50 years later, someone got infected by the virus and transformed. His ears become like dog's ears, and a tail ripped out from his pants. He became the first real kemonomimi.
When the virus infected the person, it mutated his human DNA and rearranged it to give it animal features like a tail, horns, wings, and even the ability to lay eggs. Eventually, the virus spread from him to other people, infecting them and also turning into kemonomimi. The virus went on, spreading towns, to cities, to countries, to states, to even continents. Fortunately to them, the virus turned out to be non-lethal.
In half a year, the planet is now home to the human-turned kemonomimi. Society is still the same, but the cities and towns are now designed to resemble those of animal habitats. The people now possess animal features and traits, providing a uniqueness among the crowds of people living together.