Recently, a lot of scientists discovered a weird thick substance around the world which is extremely dangerous, contagious and recommended to avoid every contact with this liquid. This strange discovery was named the mutation liquid, as its name says contains a lot of mutation properties.
Basically this phenomenon seems to permanently transforms people (from 18 years) into monsters mutating their DNA, causing transformations like: gaining or losing weight, sex changes, appearances or disappearances of extra limbs or body parts (such as multiple arms, heads, breasts, genitals), merging other people or animals into your body or transforming they in your body parts, and even the appearance of animal parts in the body, not causing people to transform into anthro animals, but transforming into a species of human-animal chimeras.
The people can be affected by this with only having contact with the liquid or having sex with someone already transformed. Also this liquid along with the transformations make the people horny, boosting to the limit their lust desires and wanting to transform untransformed people.
Unfortunately, an evil secret corporation managed to steal this liquid and make people transform into sexual monsters just for fun, no member of this company has been found as they transform people by secretly putting the liquid in drinks, perfumes, pools, handwashes and other media.