The Sigma Virus is a malicious computer virus that is yet the cognizant and ultimate form of the most malevolent cyborg "Master" Sigma as well as the new name to the corresponding infamous Maverick Virus that transforms the sentient robot species Reploids and their giant less intelligent automations called Mechaniloids into their evil counterparts known as Mavericks.
This renegade digital program has the power to infect and corrupt its targets. The Sigma Virus itself is quite diverse; not only does it cause Maverick behavior within Reploids alone, it also can make Mechaniloids go berserk with its ability. On the flipside, however, the Sigma Virus, upon infecting an already insane robot (i.e. Zero) would have the opposite effect.
Also, on its own, it can drive Reploids mad, but under Sigma's direct influence, it has also been shown to become an effective tool in subtly manipulating a Reploid's heart or even controlling them entirely as they were puppets on a string.
Once infected, the Sigma Virus is unrelenting and the vitiated cannot be easily returned to normal. Only when the host itself has died does the Sigma Virus die along with it. Therefore, the only option for most corrupted Reploids is to be "retired" by the Maverick Hunters. The exceptions to the rule that the virus dies with its host are Sigma himself, as he uses the Sigma Virus as a means to revive himself after each defeat.
At some point after the end of the X series, a cure for the Maverick Virus/Sigma Virus was developed in the form of the powerful Cyber-Elf A.I. program known as Mother Elf.