"X seem to have originated on SR388, which is also home to their sole predator, the Metroids. Since Samus Aran exterminated the Metroids on SR388, the X have overrun and infected most of the life-forms on the planet." "X infected other creatures and grow in the nervous systems of these host organisms.

The X-Parasite are a lifeform that evolved on the planet SR388. They eventually became the apex predator of the planet as no organism native to it could successfully hunt the X-Parasite, while they could prey on all of them. Eventually, the Chozo started to colonize SR388 and encountered the X-Parasite.

Each game ultimately ends with the Metroids going extinct, with them always returning too. Until Metroid Dread, where the Metroids truly are well and gone...

Metroid Fusion[edit] At the start of Metroid Fusion, whilst Samus is on a mission by the Galactic Federation to escort researchers upon the planet SR388, she encounters an X-infected Hornoad. She easily destroys it, though the leftover X-parasite flies into Samus.