No one really knows where the Canine Virus started from. By the time it was detected however it was estimated that over 80% of the human population had already been infected. At first it passed like the flu with seemingly no normal effects or even deaths beyond the occasional rare ones, but it was only the beginning of what was to come.
Infected started to lose their humanity, their feet forming into paws while their bodies got coated with fur. They stopped caring about wearing clothes and went naked wherever they went, some even doing so before their transformations really began. In the end they totally become a canine. No humanity left. There is no cure or anyway to stop it. All infected will eventually turn completely into a canine.