The X are parasitic organisms discovered on SR388 that can infect any living creature. They kill their hosts and absorb their DNA to assume the host's form. An X that has possessed multiple hosts can combine their appearances and traits during the process of replication and morph into a new entity.

The only predator, and only threat, to the X-parasites are the Metroids, who were specifically created by the Chozo to hunt and prey upon them. The X could not mimic nor control the Metroids, taking away the X's strongest offensive and defensive ability.

Samus nearly dies, but is saved by having Metroid DNA injected into her body, having lost her Power Suit as part of the process. This Metroid DNA ultimately makes her immune to infection, due to Metroids being predators of the parasite.

Ironically, Dark Samus's species of origin, Metroids, often preyed upon the species of the SA-X's origin, the X Parasites. The similarities are reinforced in Ultimate via one of Dark Samus's victory poses, which focused on her visor and showed her eyes in a similar manner to the SA-X's debut scene.