The Transformation Virus is a newly discovered viral disease that somehow transforms it's host into an anthropomorphic human-animal hybrid. Although different sources vary on its origin, the general consensus is that it started somewhere in China, where it was contained for several years before escaping.
Early cases were widely dismissed as hoaxes by experts, but as evidence of the virus mounted, the truth became clear. By the time the world was aware of the virus, it was already too late, and it had spread to every continent on the globe.
Most people who get the virus are completely asymptomatic, and never transform, but those unlucky enough to experience symptoms begin a gradual transformation that lasts for anywhere from a few days to multiple months. The transformation ultimately leaves the host as a hybrid of a human and a random animal. In some cases, around 1 in every 12, the host's sex is also changed.
Fortunately, a vaccine was quickly developed and made available to the public, but it was only effective in reducing about 2/3rds of cases. Researchers have also begun development of a cure for people in the early stages of the virus, but it is extremely unlikely that fully transformed hybrids will ever be able to return to being human. Other than vaccination, there is no known way to slow the spread of the virus