A completely unknown and technically extinct virus still had specimens frozen in glaciers south of Argentina. With the melting, copies of the virus were released from the ice and carried by the wind. The virus had been preserved for millions of years and was now on the loose, and what it could cause led to a drastic change in society. The virus itself avoided humans, parasitizing only animals, yet when it passed to humans in some way, they suffered effects that changed their lives forever.
Any animal infected by the virus never showed any symptoms and, despite it being detectable through testing, it was still a lottery to find a carrier. The virus absorbs information from the DNA of the carrying animal and, when it passes to a human, changes the person. The infection is not fatal nor does it leave the person disabled, but it fuses the genetic information of the animal from which it came with that of the infected person, transforming it into a humanoid version of that animal. The change occurred so that the infected person gained the maximum number of characteristics of the exact animal, losing the minimum of their own original characteristics, which affected the gender as well.
If a woman were infected by the virus from a male animal, her clit would develop into a penis of that specific species, normally not developing balls while her vaginas became closer to the respective animal. A man infected through a female animal would have his genitals altered by that of a male of the species, developing breasts in the same quantity as the species, ending up as a shemale.
There are two forms of contamination, which were through the consumption of any food of animal origin, such as meat, milk, eggs and the like, and prolonged contact with an infected animal.
In case of contamination due to consuming food from an infected animal, the person will have a slight fever that will pass naturally in a maximum of two days, followed by a considerable increase in appetite. The change through this type of infection is relatively quick, with the form ready in less than a month, taking longer to complete only if it is related to a large animal that makes the person grow a lot.
Contamination from living with the animal is more common if the person's pet acquires the virus, thus passing it on to its owner. In this way, the infection occurs and is absorbed by those who live with the animal more slowly, making the onset asymptomatic and the change slower, even so it is impossible to stop once it has started.