Many months ago scientists discovered a small parasite with the ability of manipulate DNA calling it the TFP (Transformation Parasite), they looked at this as a chance to study its properties and use them in humans, sadly not everything went perfect and the experiment resulted in a complete disaster. The parasite not only multiplied but attacked and killed many scientists, some of them managed to leave without injury, but the parasite escaped the lab and was able to reach the city, where everything began. Now there has been many reports about citizens being attacked by the TFP and merging with it.
The TFP's properties has been studied deeply, at the moment the information they had is that their primal instinct is to merge with a host, changing its body and gain some benefit from it, their most common form is a boneless flesh mass with a tentacle-like body where their size can vary from 2 inches to 3 feet, people merging with this type experiments drastic changes transforming into abominations. People merging with the small ones have no abysmal changes like growing extra body parts, genitals, changing their sex, size change or losing some body parts at the process with side effects like minor headaches or losing control of certain parts. The bigger the parasite the worse the transformations, people fusing with bigger ones practically turn into monsters with no control of themselves and almost or completely losing their consciousness permanently.
Another type was recently discovered called the TFP-DF, DF meaning Defined Form. As the name indicates, this type has evolved to a point where they look like real living beings like animals or plants, this one merges with a host and translates its characteristics to them, for example there was a news where a woman fishing in a river catched a big fish that resulted being a TFP-DF, the fish-like creature swallowed her legs and fused with her resulting into a mermaid, many days later she wasn't found. Also many parasites can merge with the same host resulting into a mess of chimeric being.
There has been really rare cases of some people where no matter the parasite they fuse had completely control over themselves, some of them had to get used to their new bodies but the others where consumed by lust with the desire of merging with any parasite or living being that they found.