Parasite

The parasite's skin is pale, slimy and slippery. Its body is thin and worm-like, around a forearm's length and with prominent segments like ones of a caterpillar. For limbs, the parasite has only got two spindly, hooked forelegs like ones of a spider or insect. It has a flat, rounded face with primitive yet remotely (if not unnervingly) human-like features, such as a wide maw filled with small teeth like one of a human, beady black eyes and a long, thin proboscis-tongue the length of a half of its body.

The parasite lives in people's duodenums, though it can easily move to another spot through its host’s gastrointestinal tract and, for example, start living in one's colon. To get inside its host's body, it uses a retractable stinger at the end of its worm-like body that releases a sedative substance inside the victim's veins, strong enough to make the host fall asleep and make all their muscles go slack. After the new-to-be host can no longer resist, the parasite enters their gastrointestinal system through their mouth, or in rarer cases, through their rectum. The parasite is more likely to choose an overweight person as a host.

Despite its hideous appearance, the parasite is, surprisingly, pretty harmless once inside a host's body. The parasite finds itself a kind of host to precisely stay as long as possible inside of, which explains their lack of aggression while inside one’s guts as it may inflict a life-threatening injury to their 'home'.

The parasite usually feeds on bits of digested food their host has consumed through the pylorus, with its head tucked through the sphincter and using its proboscis to suck off the remnants. It may also feed on its host's bodily fluids (blood, sweat, etc.) in other cases, but only when outside the host’s body and not causing the fluids flow itself.

If content enough writhing it’s host's body, the parasite will release a certain enzyme that is basically a mix of feel-good hormones (serotonin and oxytocin)…estrogen, and…prolactin. The enzyme has practically no visible effect on the host’s body if the parasite is single, but if there are several of them within their body, the influence of that enzyme may appear more apparently…