Maverick Virus

The Maverick Virus is a recurring virus in the Mega Man X series. It causes Reploids to disobey the three laws of robotics and go Maverick.

Origins
Before the discovery of the Maverick Virus, it was thought that Mavericks were created when the "Suffering Circuit" has errors in it.

The origin of the Maverick Virus is not explained, and is thought by many fans to have been created by Dr. Wily from the Evil Energy from Mega Man 8, or recently, the Robotenza virus found in Mega Man 10. In the X series, the Virus was unleashed when Sigma, then leader of the Maverick Hunters, investigated a red Maverick that had destroyed the entire Gamma Unit, causing Sigma himself to intervene. This Maverick turned out to be Zero.

Sigma had battled Zero in an old abandoned laboratory, where even he, the most advanced and powerful of the Maverick Hunters, sustained heavy damage. After a grueling battle, Zero had beaten Sigma into submission, and was attempting to pull of the commander's head; however, before he could, a W appeared on his helmet crystal and caused Zero to cringe in pain. Sigma, taking advantage of this opportunity, smashed the crystal, deactivating Zero and ending the fight. It is thought that at that moment, the Maverick Virus was unleashed and went inside of Sigma, who had severe injuries and was most likely weak enough to succumb to it, despite being fitted with technology that helped him from going Maverick for some time.

As the Sigma Virus
Within Sigma, the virus grew and evolved. Though created by Dr. Cain as the most advanced Reploid with technology that kept him from going Maverick, the Virus was a catalyst unlike any seen. Eventually, Sigma started the Reploid Rebellion, and recruited several Maverick Hunters into his army, both voluntary and forcibly. Though he did not start using the Virus yet, this was a foreshadowing of an ongoing war between Reploids and Mavericks known as the Maverick Wars.

After Sigma's defeat in the first game, X had thought the Maverick problem and Sigma's Rebellion was over. However, six months later, this turns out to be far from true, and X is called out into the battle field again. After battling through hordes of Mavericks, the enigmatic X-Hunters, and even a copy of Zero, a revived Sigma reveals himself, only to be destroyed one more time.

In X3, a brilliant Reploid scientist named Dr. Doppler creates a computer called the Neuro Computer, which finally neutralized the Maverick problem. Living peacefully in a gathering dubbed Doppler Town under Doppler's watch, it seemed the world was finally safe. Unfortunately, the nightmare returned when Dr. Doppler went Maverick. With Doppler Town's inhabitants on a rampage, X and Zero investigate. After destroying the eight Mavericks leading the revolt, they find none other then Sigma, who Dr. Doppler had revived. After destroying Sigma yet again, the Sigma Virus reveals itself. With the power to bend Reploids to his will, it attempted to take X's body and use it as his own. Sigma corners X, only to be saved in the nick of time by Zero (or Dr. Doppler, depending on how the game is played), who has completed a cure for the Virus.

Years later, during the events of X4, Sigma returns, planning on using a satellite weapon to attack humankind. This time the Virus is unseen, though.

However, during the events of X5, Sigma purposely loses a battle against X and Zero, spreading the Sigma Virus over a large area of the planet. Meanwhile, a mercenary named Dynamo, leading Mavericks with the Sigma Virus, intended to crash the the space-colony Eurasia into the Earth following Sigma's orders. Seeing no other choice, the Enigma Cannon is made to destroy the colony. When the Enigma Cannon fails, Zero pilots a shuttle to crash into Eurasia. Alia, a navigator new to the Maverick Hunters, reads a signature like the Sigma Virus, but with differences. This Virus is dubbed the "Zero Virus". Fighting through multiple enemies from the past, X and Zero battle, coming to a draw, and Sigma appears before them. Retreating further into his base, Sigma reveals that his plan for Eurasia's crash was to actually awaken the "true Zero", using the Sigma Virus and the Colony Virus, a variety which was used to take Eurasia off it's orbit. Sigma tells them of an old man he met, who has an undying hatred for X and knows all about Zero. Sigma is fought, and despite his defeat, manages to obliterate both X and Zero. X is repaired by Dr. Light, somehow able to appear through a mysterious force, and Zero goes missing for weeks.

During X6, the Sigma Virus is running full force, forcing the humans underground while the Maverick Hunters attempt to stop the situation. Gate, a scientist and former colleague of Alia, investigates the crashed sight of Eurasia. Finding a strange piece of Zero, a new set of Maverick outbreaks occur, and X is called out once again. In the heat of battle, a strange purple version of Zero helps him. Now called the Zero Nightmare, this being begins infecting Reploids, destroying Zero's legendary reputation. Gate leads eight researchers (actually Mavericks) to "investigate" the problem, but X goes and destroys them. The Nightmare is defeated, and the real Zero returns. Gate, angry at X's actions, directly challenges him and allows him to enter his lab. When Gate is defeated, he reveals that he not only found DNA of Zero and used it to create High Max and the Zero Nightmare, but he is also slowly, but surely, reconstructing Sigma. Fought in a half-finished body, Sigma is once again destroyed.

During X7, the Virus once again disappears, and the Earth is in repairs from the chaos of X6. New Generation Reploids are now in production, and the Red Alert Syndicate challenges the Maverick Hunters for the custody of an AWOL Syndicate member, Axl.

In X8, Sigma appears purely as the Virus. Attempting to defeat X, Zero and the now full-time Hunter Axl, Sigma is finally destroyed once and for all. However, the director of the Jakob Project, Lumine, steps from the shadows, and reveals that, having a copy of Sigma's program inside their copy chips, all New Generation Reploids can go Maverick at will.