X Parasite

The X Parasite is a parasitic organism from the Metroid universe. Appearing in the game Metroid Fusion, this life-form is capable of infecting other organisms and assimilating their genetic structure, thereby altering its own form and becoming more powerful.

Origins
The X is believed to have originated from the planet SR388, a planet once colonized by the Chozo. Their evolution and their rapid reproduction greatly upset the environmental balance of the planet, threatening total ecological collapse. Recognizing the severe threat posed by the X, the Chozo created the perfect biological weapon with which to exterminate the parasites: the Metroids. When the X tried to infect the Metroids, they were simply absorbed.

The X were driven to the brink of extinction thanks to the Metroids, but the Metroids themselves would later prove to be uncontrollable and the Chozo did not have the means of stopping their creations. The Chozo abandoned their colony and the Metroids supposedly consumed the last of the X parasites before preying on the other native life-forms of SR388, quickly becoming the dominant predators on the planet.

Resurgence
In the game Metroid II: Return of Samus, Samus Aran visits the planet SR388 on a mission to exterminate its Metroid population. At this time, neither the Galactic Federation nor the Space Pirates were aware of the existence of the X Parasites, and so Samus had no idea of the consequences that her eradication of the Metroids would have. With the Metroids removed from the food chain, X would resurface and recover, spreading once more throughout the ecosystem.

Fusion
Some years after the genocide of the Metroids from SR388, a Biologic research crew accompanied by Samus Aran conducted research on some of the native life-forms, obtaining various specimens for study. During the mission, Samus killed what appeared to be a Hornoad, but the creature was, in fact, an X mimicking a Hornoad's form. The residue from the carcass came into contact with Samus' Power Suit and infected her, but as she suffered no ill effects from the encounter, she thought little of it and continued escorting the researchers. However, the X soon infested Samus's central nervous system and later rendered her unconscious while she was in her gunship. Had it not been for the emergency systems in Samus's ship that automatically ejected her before the ship crashed into an asteroid belt, Samus would have died then and there. The B.S.L. scientists recovered Samus' escape pod and brought her to Galactic Federation Headquarters for treatment.

Though Samus had escaped her gunship's destruction, she was still infected by the X, which had now spread to various portions of her Power Suit, prompting the Federation medical personnel to surgically remove the infected parts. The infected pieces of the Power Suit were sent to the B.S.L. station for further study, where they would eventually become the monstrous SA-X. However, the medics could not remove the X embedded in Samus' central nervous system without killing her, and they had no way of counteracting the infection; Samus was given a very small chance of survival. Fortunately, the Galactic Federation scientists were able to devise a vaccine using a cell culture from the last infant Metroid. As the Metroids had been designed to be the perfect natural predator of the X, the parasites in Samus' body were quickly and completely eradicated by the Metroid Vaccine. As a side effect of the vaccine, Samus' genetic makeup now included Metroid DNA, granting her the ability to safely absorb the X like the Metroids could, but also leaving her exposed to the cold (A Metroid's weakness) as well as being much more vulnerable when attacked until she absorbed the Varia upgrade.

Shortly after Samus recovered from the infection, the Federation received a distress signal from the B.S.L. station. Samus was sent to investigate after the Federation provided her with a new suit, a new gunship, and a computerized CO to assist her in her investigations. Once Samus arrived, she found the station completely overrun by the X.

At numerous points, the X attempt to stop Samus in her tracks; they use methods such as hunting her down, mutating into newer, stronger forms, and intentionally destroying or blocking off Data Rooms to prevent her from recovering lost abilities. On one memorable occasion, the X went so far as to attempt to destroy the entire B.S.L. station along with Samus and themselves.

At the end of this adventure, Samus flies the self-destructing Biologic Space Laboratories research station near SR388 before evacuating, destroying both the space station and the planet and presumably wiping out the X forever.

Variations
Interestingly enough, they are much larger in size as opposed to the more common Green and Yellow X (as well as the rare Red X). Strangely, they are unaffected by the Power Bomb's vacuum ability.
 * Yellow X - Basic type of X. When absorbed by Samus, they restore 10 units of energy. Yellow X come out of most normal enemies. A Yellow X was the X that infected Samus on SR388 after it flew out of a defeated "Hornoad". Without her Fusion Suit, Samus would surely find absorbing these X to be lethal.
 * Green X - These appear when Samus regains her Missile Launcher and each one adds two missiles to Samus' ammunition.
 * Red X - These are usually only released when certain variations of normal enemies (usually of a different colour and harder to defeat) are killed, although they can also rarely appear from normal enemies. They are the least common non-boss X. When Samus absorbs them, they restore 500 units of health, 10 units of Power Bomb ammo and 50 units of missile ammo.
 * Blue X - X Parasites that have been changed by the cold in Sector 5 of the BSL station. Apparently, the X did this deliberately to both stop Samus from exploring the station and kill her. If Samus comes into contact with these X without the Varia Suit upgrade, she will unwillingly absorb them and be dealt a large amount of damage. This is due to the Metroid vaccine as Metroids are susceptible to ice, thus when she absorbs one of these X Parasite variants, she will be frozen from within. They are larger than most types of X, and they are the only kind that do not take the form of another creature.When Samus first encounters them in Sector 6, they are eager to be absorbed by Samus, and pursue her to achieve this goal. Samus must avoid them or else face death. She can fire her beam or Missile weapons to temporarily stun the parasites, allowing her to escape in the event that she is spotted by one. Once she reaches the Data Room in the sector and defeats the Mega Core-X, she obtains the Varia Suit. When the Varia Suit is acquired, the Blue X can be safely absorbed, and they happen to restore quite a bit of energy. Immediately after obtaining the Varia Suit, several unsuspecting Blue X will fly towards Samus in the next room. After several have been absorbed, any remaining will noticeably start to avoid and fly away from Samus, and all existing Blue X within the BSL station will now flee from Samus as soon as they detect her. They can still be stunned, and recover more health for Samus than ordinary yellow X (30 instead of only 10). These only appear in Sector 5 and Sector 6.
 * Core-X - See Core-X.

X Creatures
Most of the creatures Samus encounters in Metroid Fusion are either infected or mimicked by the X Parasites.
 * Arachnus
 * Bull Puffer
 * Fune
 * Genesis
 * Zebesian
 * Aqua Zebesian
 * Halzyn
 * Moto
 * Namihe
 * Nettori
 * Neo-Ridley
 * SA-X
 * Sciser
 * Serris
 * Skultera
 * Waver
 * Yakuza
 * Zazabi
 * Zombie Researcher