An Alien Invasion of parasitic proportions!
Because the Parasite infected his hand instead of his brain, Shinichi and Migi each retain their own, separate intellect. This gives them an edge in battling the other Parasites, who frequently attack the pair. Initially other parasites see them as an aberration that needs to be destroyed, but eventually come to see their pairing as a threat. Shinichi also feels compelled to fight other Parasites, who kill and eat humans for food.
Parasites have neither compassion for nor malice against humans. As a general rule, they display little to no emotion and act according to whatever seems sensible and likely to ensure their own survival. However, Parasites do sometimes appear shocked or surprised, and do develop individual personality traits. For example, Reiko Tamura is philosophical, Mr. A is antisocial and violent, Goto enjoys playing the piano, and Hideo Shimada prefers practicing sports. Because they are not emotionally troubled by pain or physical exhaustion, a Parasite can push its host body to the limit, performing much better than the host may have originally been capable of doing. Of course, this also increases the risk of exhaustion or injury.
Parasites have the instinctive psychic ability to sense the location and emotional state of nearby parasites by "reading" their brain waves. They are also aware of any killings committed by nearby Parasites.
Although the head of a Parasite-controlled body may appear to be made of normal human flesh, bone, and hair, it is not. Every cell of a Parasite-controlled body part is alive, and small hair and tissue samples removed from the Parasite will quickly cry out, wither, and die. Parasites can be identified by plucking strands of hair or with an X- Ray which reveals a head with no bone tissue inside. In this regard, as well as in their ability to suddenly create bizarre shapes out of normal human features, the Parasites bear a strong resemblance to The Thing . Occasionally, a Parasite displays the ability to temporarily detach itself, or part of itself, from its human host, crawling on makeshift legs to escape danger or surprise an opponent. Both Migi and Reiko have demonstrated this skill.
Parasites are very dangerous in combat. Although Parasites feel pain, it does not bother them, and they can shrug off a great deal of injury to their host body before dying. Its shape-shifting body parts can become incredibly tough and resistant to damage, deflecting blades and bullets with ease. When Parasites fight one another, they usually try to kill their opponent by severing the head from the body, or by piercing the heart (or ripping it out from the victim) in order to cause death by rapid, massive blood loss. A parasite's host body is just as vulnerable as any other; however, their resistance to pain coupled with their incredible speed makes it very difficult to hit them with a killing blow. Because all of the Parasite tissue is a "thinking muscle," they are uniquely vulnerable to injuries which damage the entire surface of the body (such as fire or acid), which can cause them extreme confusion, loss of coordination, and loss of control over their shapeshifting abilities.
Occasionally, a Parasite makes a mistake and fails to assimilate the brain of a human host. The Parasite may accidentally merge with another part of a human's body, or even accidentally merge with the brain of an animal, rather than a human. In such cases, the Parasite either subsists on the nutrient intake of the host , or has a compulsion to eat whatever animal species it assimilated, rather than humans. Such cases are seen by other Parasites as either pathetic failures or unsettling threats.
Additionally, although it is only shown in the series once, a Parasite can become partially absorbed into a human body, where in this case the human body itself becomes "super-powered", even though the Parasite still retains command over whatever part of the human body it's absorbed. Subsequently, this turns the Parasite into a symbiote of sorts, since its primary goal is to survive - which could easily be compromised should its host falls to harm. For example, with a normal Parasite, it would have taken over the head and it would be able to use the host to human limits as it pleases: with this aforementioned method, the Parasite still regains command, but now the human body can far surpass human limits, to almost super-human levels. Only one character in the series ever has this happen to them, so it could be a total fluke.