It was finally the Goddess of Horses that won the day. Her Unicorns had long protected the forests of the world, peaceful and focused on ensuring nature would persevere. She proposed that a virus that would turn humans into her children would keep them from repeating the past mistakes. As such the decision was made to turn them into Equines, not just Unicorns but Pegasi, Horses, Zebras, and other species of equine.
The transformation process would be slower than other types of infections. In the first stage the infected would start to grow fur across their body and expand to closer to equine proportions. In the second stage, their features would start to become more equine, their genitallia changing along with growing wings or horns if they are unicorns or pegasi. At the third stage their bodies will fully remble anthro equines and completely refuse to wear clothing, also they would begin to show magic at this phase. Finally in stage four they will have nothing left that resembles humanity, going fully equine. This process can take anywhere from days to weeks depending on the strength of the immune system.
The virus would spread through sex and body fluids during most of the infection stages. Once the infected goes full equine however, the infection will become airborne. The possibiility also exists for it to jump species into natural born equines, granting them human level intelligence.