The Parasite

The Parasite is a disease that infects one person at a time and can only spread to one other person when leaving its host. In the world, there are currently 50 holders of The Parasite. Once someone has been infected, they're marked with a black tattoo-like mark across their back.

Disease Effect
While The Parasite has infected a host, a black tattoo-like mark will spread across their back over the course of 3 days. Once the mark is complete, the host will find they've grown invisible arms (dubbed "Vectors") out of the center of this mark. These vectors cannot be unsummoned, and will always follow the person around. This can cause the person to become extremely clumsy, in Liliana Parellian's case, where her high number of vectors causes her to trip and knock things over often.

The Mark
The mark that grows on a host's back is always completely black and shaped geometrically. The mark will start as a circle in the center of the host's back and extend in branches from the circle. The number of branches always reflects the number of vectors. Commonly, these marks will extend along the host's arms and at times the front and sides of their torso.

In Liliana Parellian's case, where she has 50 vectors, her back does not have the simplistic circle and branches, but a rather intricate design that does not extend to her arms and instead covers the front of her torso.

Vectors
The vectors come in a specific number depending on both the host and the version of Parasite. The smallest amount recorded was 2, and the highest number seen is 50. It is possible that vectors remain around the same count between family members due to similar DNA and how easy it would be for a single Parasite to pass between family members.

Vectors are best described as invisible arms, and they act similarly to them as well. They can not pass through objects, but they can pass through organic matter. Vectors can be used for combat easily. as the host can choose whether or not they want their vectors to pass through organic matter or not at the time. However, no matter what the host wants it they can never pass through objects - including walls, floors, and ceilings. The host can still pick up and use objects with the vectors if they so choose.

Despite a high number of vectors being extremely powerful, the more vectors you have the more tiring it is to use them. It would not matter if the host chose to use only a fraction of their vectors, if they had a high amount it would be equally as tiring as if they were using all of them.

If the host wishes to rid themselves of the Parasite, they can focus all their vectors onto one single person, and pass the Parasite to them. This does not rid them of their mark, but it does rid them of their vectors.

Side Effects
Once The Parasite leaves a host, they'll be left with a random side effect. It is unknown what exactly determines this side effect, but there has never been a case where it is not detrimental to the person it has infected. It is also unknown where this disease originated or how to cure it.

Note: The Parasite is based on the ability diiclonius have within the anime/manga "Elfen Lied", however it is used in a different way and not an exact rip from the series.