"Parasites" are tennis ball sized creatures, whose numbers and origins are unknown. They invade and take over the human mind in order to survive. Shinichi Izumi is a 16-year-old high school student who lives with his parents in a quiet neighborhood. One night, a Parasite invades Shinichi's body in an attempt to take control of it. However, it fails to complete the takeover process and ultimately ends up inhabiting only his right arm.

Thus, both Shinichi and the Parasite—who calls himself Migi—retain their individual minds. Now in a peculiar "human and alien" relationship, Migi proposes that they cooperate to survive. Shinichi has no choice but to agree, and he must now cling to his morality as he and Migi face off against other bloodthirsty Parasites.

Kiseijuu won the 17th Kodansha Manga Award for Best General Manga in 1993.

In 2003, it was reprinted in a eight volume complete version format; in 2014, in a new edition format, and complete version in a bunko format. The manga was adapted into two live-action movies which released on November 29, 2014 and April 25, 2015, respectively. A spin-off live-action series titled Parasyte: The Grey was released on Netflix in 2024.

The series was first published in English as Parasyte by Tokyopop, ran in their manga anthology magazine Mixxzine and had a 12 volume release from June 23, 1998 to November 19, 2002. Del Rey Manga republished the series based on the eight volume reprint from May 1, 2007 to July 28, 2009. Kodansha Comics USA later reprinted the Del Rey release from July 26, 2011 to August 21, 2012. It has been published in Spanish by Planeta Comic and was published in Brazilian Portuguese by Jbc from September 2015 to June 2016. The manga was also published in Italian by Magic Press from October 3, 1998 to August 30, 2004 before release was discontinued. RW Edizioni picked up the license and published the manga based on the eight volume reprint under the GOEN imprint from December 6, 2014 to January 30, 2016.