The X Parasite (also known both singularly and collectively as "X") is an alien creature found in the Metroid Universe on planet SR388 and on the BSL research station. It is not a true parasite as a parasite does not kill the host unless absolutely necessary. It is an ectoparasitoid. In its natural state, an X parasite is a free-floating glob of protoplasmic material. It invades the host's body, absorbs its DNA and intelligence, and kills the host by converting its body to X material. An X can remain in a host's form even after the host's death, and while sufficient damage will force it to revert to its normal shape, it can later revert to a previously-acquired form. Some X have even demonstrated the ability to combine different host DNA into a new creature.
Many X can come together and form one creature called a Core-X. The change is not chemical however and individual X can be forced from the Core-X.
The X have one predator, the Metroid. They were genetically engineered by the Chozo to destroy the X Parasites and are immune to X infestation: an X attempting to infest a Metroid will instead be consumed. After Samus Aran exterminated the Metroid race, the X population quickly multiplied. Despite their virulent nature they managed to hide their take over of the planet from the scientists in system until Samus Aran was attacked and infected by one.
In addition to those previously mentioned abilities the X has several unusual abilities. Its ability to absorb intelligence also allows it to organically absorb and implement data, for example to create a heat resistant material to wrap itself in. It reproduces through a process similar to viruses, which makes it almost infinitely adaptable. When an X doesn’t come into contact with a susceptible host it stops growing and reproducing which means it needs no energy for food. It can float forever until disturbed. This makes it amazingly adaptable and incredibly hard to safely breed in laboratory conditions. They are also extremely and self-destructively aggressive, as they will even charge a foe it cannot hope to kill.
The X Parasites are such prolific breeders that on their home planet of SR388 they threatened to destroy the ecosystem. The Chozo stepped in and created the Metroids in order to keep the X Parasites' numbers in check. When series protagonist Samus Aran exterminated all of the Metroids on SR 388 in Metroid II: Return of Samus, the X Parasites' population quickly made a comeback. They provided the main antagonist role in Metroid Fusion, particularly in the form of the SA-X, an X mimic of Samus Aran and her Power Suit. As the X continued to menace the station, the Federation took an interest in the X. They ordered Samus' computer to confine her until Federation ships arrive. Samus, knowing that the X would overwhelm them and spread through the universe, set the station on a collision course with SR-388. By doing so, all of the X on the station and the planet would be destroyed. The SA-X tried to stop her, but was defeated.
The SA-X is an X Parasite that copied Samus Aran's abilities on one of her trips to SR388 and dwelled within her until it was surgically and biologically removed. The SA-X's name means that is an X copy of Samus. The letters S, A, and X stand for Samus Aran and X (as in "X Parasite" or representing an unknown or different version), respectively. The infected Power Suit was transported to a B.S.L. Research Station in orbit above SR388. But it escaped its containment cell using one of Samus' Power Bombs and set free all the X Parasites in the Quarantine Bay. Samus was sent to the station to find all crew members dead and the X infesting several sections of the station. Her initial investigations accidentally allowed the X access to some of the breeding environments by releasing security locks. Subsequently, the SA-X manipulated Samus by destroying doors and wreaking havoc in certain areas to release all the security locks and allow the X Parasites access to all breeding areas. Though ADAM warns Samus of ten SA-X aboard the station, only two are seen - one is destroyed when the Restricted Area self destucts; the second confronts Samus after she set the station on a course to hit the planet SR388, and later returns to try and defeat the Omega Metroid. The Omega Metroid defeats this SA-X which subsequently provides Samus with her coveted Ice Beam to defeat the Omege Metroid, in a similar fashion to the Hyper Beam from the ending of Super Metroid, which was used to defeat Motherbrain.
During the course of the game, Samus discovers a secret laboratory on the station that is cultivating Metroids, which the SA-X promptly destroys. After the lab is detached from the station with the SA-X on board, Adam, Samus' computerized CO, explains that the SA-X has been reproducing and that now there are no less than ten SA-X onboard.
The SA-X is extremely dangerous in close combat, mimicking Samus at the height of her powers. It is equipped with the Screw Attack and Space Jump, and fires beam shots appearing to be a combination of the Plasma, Ice, and Wave beams. It cannot be damaged by Samus until the final fight against the SA-X.
Samus prevails over another SA-X in close combat, which morphed into a large monster after several Charge Beam shots. After the SA-X physical form, monster (which is thought to transform into a hornoad-like creature because Samus absorbed an X from a hornoad while still wearing her original power suit, thus making the X Parasite still have hornoad DNA in it.) and Core-X are destroyed, the parasitic SA-X escapes before she can absorb it (as she did other X Parasites). This SA-X resurfaces a short time later, after Samus had set the B.S.L. station on a collision course, during Samus' final battle with the Omega Metroid, when Samus is about to be killed. The SA-X fends off the Omega Metroid but is then fatally wounded, returning it to its parasitic state (some suggest that the SA-X came back and fought the Omega Metroid because the X were hunted by Metroids). It is then absorbed by Samus, which fused with her original power with her new fusion suit abilities and completes her. This new suit, the Omega Suit, now colored in her trademark orange, yellow, and red, also finally gives back Samus' Ice Beam. Using her newly restored power, she defeats the Omega Metroid using the Ice Beam and clears the way. The computerized CO, with the help of the Etecoons and Dachoras, pilot Samus' ship to pick her up before the station crashes into SR388, eliminating any X Parasites on it (and therefore all the existing SA-X).