Following through with the disposal of Mavericks, the human leadership decided to have Mavericks exterminated by other Reploids and formed the Maverick Hunters. Driving robots mad locally, the human leadership acknowledged these infected robots as Mavericks, and ordered them to be disposed of. Surviving from over 100 years ago, an unknown computer virus emerged from the capsule Zero slept in. At some point after the end of the X series, a cure for the Maverick virus was developed in the form of the Mother Elf. The exceptions to the rule that the virus dies with its host are Sigma, as he uses the Sigma Virus as a means to revive himself after each defeat. Therefore, the only option for most infected Reploids is to be retired by the Maverick Hunters.

Only when the host itself has died does the Sigma Virus die along with it. Once infected, the Sigma Virus is relentless and the infected cannot be easily returned to normalcy. Also, on its own, it can drive Reploids to insanity, but under Sigma's direct control, it has also been shown to become an effective tool in subtly manipulating a Reploid's heart or even controlling them completely like puppets on a string.

On the flipside, however, the Sigma Virus, upon infecting an already insane robot (i.e. The Sigma Virus is quite diverse not only does it cause Maverick behavior in Reploids, it also has the ability to make Mechaniloids go berserk. As a virus, the Sigma Virus has the ability to infect and corrupt its targets.

Mega man project zero zigma