No one really knows where the Equine 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 hooves 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.
The infection has different stages:
Stage 1: Initial Infection
The virus acts like the flu or a cold, victims are the most infectious at this stage, often infecting their entire family and friends before the disease runs its course.
Stage 2: Gestation Period.
No further signs of infection are present during this stage, however infected can start to care less and less about wearing clothing. Infection is typically only done by coming in contact with body fluids at this point.
Stage 3: First Changes.
Infected start to grow fur and undergo other small transformations. They usually are small enough to not be noticeable if the infected wear clothing.
Stage 4: Changes Intensify.
The changes start to increase at this point, usually this is when genitalia, extremities, and other body parts transform. Tails will also grow during this period.
Stage 5: Full Anthro.
Infected in this stage completely give up on wearing clothing having transformed into fully anthropomorphized equines. Infected usually give completely into their new instincts however maintain their human intelligence usually. They also have a growing desire to infect others by any means necessary.
Stage 6: Final Form.
Infected in this stage no longer resemble humans having been fully transformed into equines. They tend to retain human level intelligence but almost never remember being human to begin with. The equine instincts are now all consuming and they want to infect any uninfected humans.
In addition to the stages there are several strains of the virus that determine what you are transformed into over the course of the disease.
Horse Strain:
The most common strain, infected turn into a horse of any number of breeds.
Donkey Strain:
The second most common strain.
Zebra Strain:
The third most common strain, its speculated that this one began in Africa but no one is sure.
Mythical Strain:
The rarest of the strains, it will come up on occasion and the infected will become a Unicorn or a Pegasus.
The virus can also infect normal equines and give them human level intelligence.