After Samus’s power suit and central nervous system were overwhelmed by X parasites, the surgically removed parts of her suit were immediately quarantined and sent back to the Biologic Space Labs for further research while Federation scientists searched for a vaccine.
It appears that Federation HQ had not yet learned the full extent of the X’s ability to quickly propagate and biologically mimic a host, including the parasite’s frightening ability to absorb genetic information, memories, and even evolve to organically process machine data. Coupled with the X’s instinctive hunger for power, the parasites contained within Samus’s infected suit parts were able to reconstitute themselves into a physical form based on Samus and her powerful arsenal.
The “SA-X†is the abbreviated name given by ADAM to describe these lethal life-forms. As Metroids are the X parasites' natural enemy, the SA-X’s primary weapon of choice is a highly powered Ice Beam, which has a potent effect on Samus due to her recent Metroid-derived vaccine. In other circumstances the SA-X will use Screw Attack or overpowered Super Missiles to destroy doors and other structures.
While only one SA-X is seen at a time, ADAM suggests later in the mission that these fearsome X entities have replicated themselves with no fewer than ten different SA-X simultaneously roaming the space station. Safely traveling the B.S.L increasingly becomes a challenge for both Samus and her CO, who is forced to make strict navigational decisions.
