The world secretly was affected by a mysterious virus that came from nowhere that later was discovered, it was called the body part transformation virus or also known for its scientific name BPV-25.
The name was self explanatory, this disease that managed to infect people made them to turn into someone else’s body part after physical contact, fusing with their already existing ones but adding extra mass, becoming an additional one or merging into something unexpected, the results could be entirely unpredictable. Many cases like that have been seen like turning into someone else’s third arm, becoming extra mass in a woman’s butt or even some occasions of people fusing with someone to become a “humantaur” as their lower half.
But the other thing is that was discovered that there were two versions of the virus, the “dominant” and the “submissive” version. The people carrying the dominant one could cause people merge with them, even turning uninfected ones into their body parts, being able to turn one person or much more into their parts. The submissive on the other side was the opposite, making the ones infected by this version likely to transform into someone else’s body part even into the one who is not infected. As previously mentioned all of this is unpredictable, there are chances that a person infected by a dominant BPV-25 rarely merges someone with their body or even never doing it, the same with the submissive virus, but if a dominant and a submissive carriers have physical contact with each other there’s a 100% chance that they will merge.
This virus have led to many problematic situations due its chaotic results and the people infected having barely any symptoms making it almost impossible to track and prevent the transformations. When the person is already merged with someone else they could experience symptoms like a higher libido or the desire to keep merging, some don’t experience that kind of feelings and can keep living their “normal” life with their new body, and others even enjoy their new form. There are rumors about finding a cure but the future for it is not certain, sometimes looking like it’s decades away.