|Anime debut||Episode 1|
|Manga debut||Chapter 1|
Parasites are an unknown species and are the primary antagonists in the Parasyte series (excluding Migi and Jaw). They are a mysterious species that appeared on Earth from an unknown cause. Their origins, to this point, still remain a mystery.
Parasites go through two main stages of development: a larval or primal form and a matured, humanlike form.
The larval form resembles a green, almost translucent worm or snake with yellow eyes protruding out of its head, and a drill-like appendage on the front of its head. In the anime, they're small, approximately the length of a finger, while they are two or three times larger in the manga. The eggs or spores they emerge from resemble green airborne tennis balls filled with goo.
The second stage is when the parasite finds its desired host and enters its body, maturing in a matter of minutes inside to become in sync with the host and replacing the body part it inhabits. Parasites have skin resembling their host and insides to match, which can be changed as they desire. They can morph into various shapes and extend and harden certain parts of their body as they desire. Due to their instincts, most will be in the shape of a human head by default.
A larval parasite instinctively aims for a human host's brain. Parasites that are unable to take over a brain or end up in an animal host or any other part of the host's body are considered failures.
After taking control of the brain of a human or an animal, parasites develop the instinct to kill and devour other members of their host species (parasites who took over humans desire to consume other humans, while those occupying dogs desire to consume other dogs), as well as destroy or devour parasites who didn't manage to take their host's brain but their other body parts. However, if a parasite does not take control of the brain of any species, it does not develop this instinctive cannibalistic hunger or violence. Despite these strong instincts, many parasites can ignore or outgrow their desire to eat humans.
Parasites are also only capable of occupying bodies of their first host species. Any new hosts must also be of the same sex as the original, or they will be unable to control the new sex organs and the new host bodies will reject them. Despite this, parasites do not seem to be attached to their original host's gender: the only cross-gender swaps shown changed physical appearance to mimic its new hosts, as seen when the female parasite that took over Nobuko Izumi swapped from a female to a male face and body, then to a female body again in short succession.
Although they can live off of the food that their controlled organism would, they usually choose not to. This trend does begin to change as they continue to develop, their murders dwindling as some choose to adapt to human society and decrease or entirely cut out humans from their diet in exchange for safety from being discovered.
It was explained by Dr. Yui that a parasite's cells double as both muscles and neurons (i.e. "sentient muscle"), meaning that their flesh also doubles as their brain. This was confirmed by Migi during his "division demonstration" and each time he divided himself, his intelligence dwindled.
Parasites also often possess a great degree of knowledge: they may use books, television, and the Internet to acquire the ability to speak as well as a great amount of background knowledge about the world, which they use to their advantage. They adapt swiftly, often learning the surrounding language in a matter of hours to days. Some parasites can use their intelligence to become leaders of other parasites, and relay instructions and plans to 'lower' parasites.
Parasites rarely feel or express strong emotions and usually must fake them if needed, though among the list of naturally-expressed reactions include rage, mocking amusement, curiosity, shock, and despair at losing a host body. Even when feeling such emotions they often do not facially react the way a human would unless they consciously choose to, such as laughing without smiling. Two other parasites have been shown to gain more "human" emotions such as empathy or familial love, but both have gone through a change that brought them closer to being human; Migi fused with Shinichi's heart, and Reiko Tamura gave birth to and cared for a human child. Currently, no parasite has displayed romantic love or lust, though they have shown scientific interest in human and parasite reproduction.
Despite this apparent lack of humanity, they can instinctively recognize others of their species by the signal they all emit and regard them as allies rather than competition. Even completely inhuman parasites are willing to work together for unknown reasons, such as A and Hideo Shimada with Reiko, while Jaw with Mamoru Uda teams up with Shinichi and Migi for the simple reason that they are alike.
Parasites regard names and identities as unimportant, freely switching both appearance and identifier whenever it would be inconvenient to be recognized. If prompted to rename themselves they may take a different but similar name, such as the case of Reiko Tamura, a placeholder name such as A, or a straightforward name such as Jaw.
Though only two cases have been witnessed (Migi and Jaw), it's notable that parasites who fail to take over the brain of their host appear to lean towards being of the same gender as said host. Also, due to the human brain still being intact, parasites who fail to take over the brain are seen as outcasts or dangers and often end up targeted by man-eating parasites who discover them. Parasites who act differently from the norm may likewise be targeted.
When congregating away from humans. They sit in haphazard order and sometimes use their ability to morph extra facial features to look directly at relevant speakers without bothering to turn their bodies to face them. Similarly, when alone they may morph tentacles or other things to manipulate objects instead of using their human body.
While answering a letter, Hitoshi Iwaaki stated that a parasite usually splits its head open while feeding, but will unravel its head in a spiral pattern when on the offensive, due to being quicker.
Abilities & Powers
The larval parasite has only the power to float in the wind while still in its egg. Once hatched, its head can be used to drill into a human body, leaving no marks or blood behind.
All matured parasites have the ability to morph into many different sizes and shapes and can become elastic or harden into a substance stronger than steel. Even seemingly untransformed parasite parts are incredibly strong, capable of crushing bones or breaking metal. They can transform their faces and other physical characteristics like hair or even mimic artificial objects. A parasite that did not consume a brain can still mimic a human face quite closely if necessary, though it would be unable to actually control a host body. Their ability to morph is most often used to blend in with human society (though their empty gaze and lack of empathy can distinguish them from certain humans) or fight and kill humans or other parasites. Because of their morphing powers, parasite flesh can survive being stabbed, slashed, crushed, or otherwise detached so long as they have a source of blood or return quickly enough to the main mass. Parasite-occupied bodies possess physical capabilities that are exponentially greater than normal humans' because they can use the human body's full potential and are not intimidated by pain, though this can also backfire by damaging the body if they use more strength than it can withstand.
They can identify other parasites through the wavelengths emitted by their brains. Weakened or sleeping parasites emit weaker signals. Consequently, parasites are able to sense the mood and activity of others of their species, even using this sense as a crude communication channel. Although this signal is somewhat imprecise, they are also capable of identifying other parasites or parasite-bearers in crowds simply by meeting each other's gaze. Most importantly, they can sense each other's killing intent.
Certain parasites have been shown to control other parasites, most notably Gotou who controlled a body containing four others of his kind (up to five with Migi). This ability is shown to require great focus as Miki could only control three, including himself. It is currently unknown whether there is a mental or physical limit as to the maximum amount of parasites able to be contained and controlled within a host.
Parasites also possess enhanced reflexes and perception, as shown throughout the series. As Shinichi tries to stab Migi when the latter was just matured, he quickly reacted by blocking the knife and broke the blade. Before being enhanced, Shinichi was unable to track parasite attacks due to their sheer speed. Some parasites can also use multiple different attack patterns simultaneously to increase their lethality, as seen when Ryoko intimidated Migi.
Despite instinctively wanting to eat the species of their hosts, parasites can live off a non-cannibalistic diet and only need food that will sustain their host bodies. Parasites may also consume or be sustained by blood, and maybe capable of revitalizing themselves by eating human prey if their own host body has lost a lot of blood. They can also detect the condition of their host bodies, such as when Migi stated appreciation for a healthy meal that Shinichi had eaten and Gotou's fellow parasites detected poison in their shared body.
The biggest weakness parasites possess is their dependence on the organs and digestive system of the host's body: once cut off or separated, in a matter of minutes a parasite will begin to weaken, shrivel and eventually die. If their host body is fatally exsanguinated or rejects them after a transfer, they will begin to shrivel away even while still attached and will require to transfer to another host in order to survive.
Another weakness is their intolerance to fire. Migi stated that fire causes their surface cells to react in a way similar to "freaking out". This weakness was exploited during Shinichi's fight with Gotou, causing him to become distracted and unbalanced.
Parasites are also affected by toxins, diseases, and infections. Alcohol has shown to be a danger to them, causing their morphing abilities to be harder to control if ingested, while more caustic chemicals like paint thinner can damage their cells and cause them to lose their morphing capabilities in the affected area as well as divide their body into smaller, uncontrollable masses.
They become less capable as their cells are split apart or damaged, with smaller pieces quickly losing the capability to think and only following their last "command" or struggling to reconnect with the main mass. If a parasite detaches enough of themselves they will eventually lose control of their host body. They seem slow to regenerate, as Migi did not recover from his handicap after using part of his body to heal Shinichi and appear incapable of producing more of their species.
The host's body is also an inhibition with combat, as the body is easily damaged and can also be destroyed by the occupying parasite if it tries to fight using it, most notably when A easily broke his wrist when attacking a group of teachers.
- Migi - Alive (Dormant)
- Jaw - Alive
- Parasite Dog - Deceased
- B - Deceased
- A - Deceased
- Hideo Shimada - Deceased
- Reiko Tamura - Deceased
- Kusano - Deceased
- Hikawa - Deceased
- Maesawa - Deceased
- Kawada - Deceased
- Miki - Deceased
- Gotou - Deceased
- Nobuko Izumi (Parasite) - Deceased
- Fujii - Unknown
- Numerous unnamed/unseen parasites - Some Deceased/Alive
- They are nicknamed "Mouth-heads" by civilians who happen to spot them.
- While answering fan mail, Hitoshi Iwaaki hinted at the origin of the parasites, saying the "eggs" seemed to have been carried by the wind and that they appeared to have come from somewhere on Earth.
- In other fan mail, he has also referred to them as social animals and a lot like humans.
- The only parasite in nature that resembles Parasites in the series (in behavior and function) is Cymothoa exigua A.K.A. the tongue-eating louse. It is a fish parasite that first eats the tongue of a fish and then uses its body to replace the consumed body part, while the fish remains alive and relatively unharmed.
- Parasites have a similar function, though they eat and replace the host's brain. But Migi and Shinichi share a similar relationship, wherein this case, the parasite consumed and replaced a body part, while the host remained alive and intact.
- In the original Japanese, the only time the titular Kiseijuu are mentioned is at the end of Takeshi Hirokawa's speech, implying the title of the work refers to humans. Parasites are instead referred to as "Parasaito" in katakana and occasionally "kiseiseibutsu" (寄生生物), meaning "parasitic lifeforms".
- The parasites are similar to "The Thing", since they are parasitic life forms that can manipulate their bodies in a way similar to the parasites from the series.
- When entering the defensive mode, the parasites heavily resemble the Las Plagas parasites from the Resident Evil video game series. This is because the concept of these parasites was inspired by Parasyte. Like them, Las Plagas will erupt from the host's head when defending both itself and the host body, under attack. It sprouts tentacles, multiple, jumbled eyes, and even a single tentacle with a bony, scythe-like weapon at the end, greatly resembling a parasite's.