The Parasite (Eucestoda horriblis possidendam)

Summary
Eucestoda horriblis possidendam, colloquially referred to as the possessing tapeworm or just "The Parasite", represents the root cause for the dying and turning of humans infected by its larvae. They are characterized by their exceptional mutation rate, a factor which is thought to be the main reason why the ordinary tapeworm could evolve into the parasite so quickly and unexpectedly. Human involvement in the creation of Eucestoda horriblis isn't suspected.

Biology and Turning Process
Upon transmission of the semi-developed larvae by a previous host either through a bite or a sufficiently severe wound, the larvae locate the nearest vein and move towards the human brain. When at its destination, the baby-tapeworm first leeches at the brainstem for approximately 12 up to 48 hours, collecting nutrients and starting its special form of metamorphosis, ultimately transforming from larvae into the full parasite. When ready, the parasite's body splits at predetermined locations, allowing it to grasp onto the brain beneath the brain sac, disrupting and interfering with the host's nervous system. Eventually, and if the procedure succeeds, the parasite severs the brains blood flow, consequently killing the host, who appears to be traditionally dead for a certain duration.

During this timespan, the parasite gains control to all major arteries, creating its so-called Endo-Neurosystem inside the human body. Unnecessary brain matter such as the cerebellum (responsible for coordination) and the parietal lobe (responsible for elaborate speech and the ability to make sense of one's surroundings) are completely dissolved and its proteins are used for the generation of this Endo-Neurosystem. Oftentimes, the occipital lobe, responsible for vision, is also severely damaged, so that the Host's ability to see objects and threats is impaired in some shape or form. Upon finishing, the parasite will have gained a conection to all major muscles necessary for moving and will substitute a minimal digestive system for staying alive. The main and original part of the parasite will still sit at the brain stem, where it controls all basic vital functions of the body for the time being, albeit extremely inhibited.

The transformation process typically ends with the patient first experiencing twitching movement in their extremities as the parasite gets a feeling for the body it's gained, followed by standing up and "doing what a zombie does", as the transformation is completed. After that, the main goal of the fused host and parasite, simply called an infected, is to transfer its offspring to as many victims as possible. Upon transmission, the infected and the parasite is awarded with a form of sexual arousal, incentivicing it to repeat what it has just done.

Limitations
A connection or parts of the parasite's system may be damaged, severed or not even formed during metamorphosis, leading to the parasite being unable to control and/or use said bodypart, leading to the creation of a so called Crippled. These evolutionary mistakes typically doesn't pose a big threat to living humans, and will perish timely unless it experiences ideal conditions. In contrast, the parasites sometimes experiences spontaneous mutation either during metamorphosis or its asexual reproduction, leading to hosts developing special, oftentimes beneficial abilities along the way. If the mutation is evolutionarily successfull, it will allow the mutated infected to transfer its parasite's larvae to a larger amount of victims, thus increasing the spread of the mutated form.

The typical mutation will have very limited access to the vision complex of the body, so it needs to substitute other senses for its lack of vision. As a result of that, hearing of most typical infected is extremely sensitive, in contrast to atypical infected that retained the brain part responsible for their vision.

The parasite's larvae can't endure exposure to air for an extended period of time, thus the transmission has to be near instantaneous for the larvae to survive. As a result of that, bites are the most effective form of transferring larvae. Still, even lifeless parasites can pose a severe danger to humans, namely due to bacterious infection, ultimately still leading to the victim's demise as other infected come to feast on the corpse and transmit the larvae into it.

Transmissions in the animal kingdom have been observed, but the success rate seems to be significantly lower compared to humans, which turn almost 85% of the time.

Eucestoda horriblis possidendam, when having aquired its final Host possesion form, is very suceptible to air moisture levels, since the host body looses the ability to drink water in order to supply the parasite. For that reason, Hosts display visible lethargy when the air is very try, contrary to the visiousness that can be observed in areas which contain a higher level of moisture or during rainfall.

First perceived by the general public to be a virus,