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The Sigma Virus (シグマウィルス), originally known as the Maverick Virus (イレギュラーウィルス) up until Mega Man X3, is a dangerous computer virus in the X era that can force Reploids to become Mavericks. It was originally created by Dr. Wily, only for Sigma to later absorb and assimilate it into an extension of his own malevolent will.
In the Games
In Bass's ending in Mega Man 2: The Power Fighters, Dr. Wily shows Bass the plans for Zero, his ultimate robot. When Bass scoffs at this project, Wily mentions "another project", assumed to be the Maverick Virus. However, this game's story is allegedly set in an alternate timeline, making the canon status of this interaction debatable.
The history of the Maverick Virus is only addressed in production notes and supplementary material, and never directly explained in the Mega Man X video games. The virus was created by Dr. Wily, designed to infuse robots with evil and destructive impulses. When constructing Zero, Wily found an irreparable glitch that made him uncontrollably violent; as a solution, he resolved to infect him with the Maverick Virus to flush his system and ensure his obedience. However, nearing the end of his life, Wily never perfected the Maverick Virus, leaving both of his final creations hidden in a laboratory.
Zero eventually awoke on his own, and the Maverick Virus somehow leaked. During their fated battle (depicted in a flashback in Mega Man X4), both Zero and Sigma became infected by the Maverick Virus. The two Reploids had very different reactions. Zero was pacified and lost his psychotic bloodlust, along with his memory, turning him into a reasonable individual. Sigma initially exhibited no adverse symptoms due to his highly-advanced mental circuitry, but the Maverick Virus secretly festered in him. Over time, Sigma's systems assimilated the Maverick Virus, infusing his very Reploid soul into the virus. Though it wasn't yet called it, this was the inception of the Sigma Virus.
Mega Man X
In Mega Man X, the Sigma Virus is only briefly alluded to in the game's ending. After his defeat, Sigma's final message boasts that his "soul" lives on, hinting at the existence of the Sigma Virus and Sigma's ability to live on through it.
Mega Man X2
In Mega Man X2, the Maverick Virus is first mentioned as a virus that Magna Centipede is transmitting around the globe from the Central Computer Stage. The sub-boss in that stage, Chop Register, is said to be a manifestation of Sigma's swordplay data via the Sigma Virus.
During the final battle at the Central Computer Stage, X destroys Neo Sigma's body and reveals his true form: the Sigma Virus. The Sigma Virus fights by drawing on the power of the Central Computer to fire beams and summon Maverick Mechaniloids - as X damages it, it uses more power. The battle ends just as Zero destroys the Central Computer, forcing the Sigma Virus to fade away, but before disappearing he mysteriously claims that Zero is destined to become Maverick, hearkening back to his original purpose from Dr. Wily.
Mega Man X3
In Mega Man X3, the virus is no longer referred to as the Maverick Virus, only being called the Sigma Virus. It seems Sigma Virus-infected Mavericks have become commonplace thanks to the events of Mega Man X2, as Dr. Doppler researches ways to suppress Maverick behavior. Once he succeeds, he saves many Mavericks and together they form the utopia Dopple Town - that is, until Dr. Doppler becomes infected by the Sigma Virus himself and starts a new rebellion. He also infects many of the city's visitors, such as Blast Hornet and Tunnel Rhino, to get them to join the Doppler Army. Much like Chop Register, the sub-boss Shurikein is also said to be a manifestation of the Sigma Virus. The English game manual states that Vile MK-II is infected by the Sigma Virus, but this has never been stated elsewhere and may be non-canon.
After X defeats Dr. Doppler in Doppler Stage C, he somehow regains control and urges X to go defeat Sigma, who had brainwashed him into starting the rebellion. Once X destroys Kaiser Sigma in Doppler Stage D, the Sigma Virus emerges and pursues him through the burning laboratory, threatening to take his body. Depending on the player's actions in the game, either Zero or Dr. Doppler arrives with a vaccine to the Sigma Virus from Doppler's laboratory. They strike the Sigma Virus with the vaccine, causing him to fade away as the lab collapses.
Mega Man X4
In Mega Man X4, the Sigma Virus is not mentioned at all, besides the flashback showing the battle in which Zero and Sigma were infected by the Maverick Virus. Sigma achieves his evil aims through manipulation and deceit rather than infection, making it seem as though Doppler's vaccine had some effect on his viral influence.
Mega Man X5
In Mega Man X5, the Sigma Virus returns as a major plot element. When X and Zero defeat Sigma Head in the Opening Stage, his explosion makes the Sigma Virus start spreading across the entire planet, causing mass panic. Simultaneously, Dynamo infects the space colony Eurasia with a virus that sends it on a collision course with the Earth. Throughout the game, X and Zero race across the globe to collect the parts needed to stop the crash of Eurasia, facing bosses who have succumbed to the influence of the Sigma Virus. Additionally, X and Zero can be infected by floating Sigma Virus drones during missions: once they reach their capacity, X begins taking damage, while Zero actually gains a power boost.
When the Eurasia colony crashes, completely or not, its Colony Virus mixes with the Sigma Virus, creating a super-powerful virus known as the Zero Virus. Depending on the player's actions, Zero may be infected by the Zero Virus in the process, entering his Awakened Zero form as originally intended by Dr. Wily. Sigma later explains that this was his plan all along, as he sought to pit X and Zero against each other. He also mentions that he received this information from an "old man" via the Sigma Virus, implying that Sigma was somehow able to communicate with Wily through their shared connection to the Maverick Virus. Despite this, X (and potentially Zero) defeat Sigma within the void of Zero Space, which results in the Sigma Virus and Zero Virus completely disappearing.
Mega Man X6
In Mega Man X6, the Sigma Virus is only briefly mentioned, as Gate used data on Zero's DNA and the Sigma Virus to create a new virus called the Nightmare. Although Gate also resurrected Sigma, it is unclear if Sigma still has access to the Sigma Virus in his withered form.
Mega Man X7
In Mega Man X7, the Sigma Virus is only briefly alluded to. When confronting Flame Hyenard, X hypothesizes that his bizarre behavior is a result of the Sigma Virus. Later, it is revealed that Sigma took control of Red Alert's members when he gave them their DNA power-ups, though it is left unclear if he did so using the Sigma Virus or another method.
Mega Man X8
In Mega Man X8, the Sigma Virus is again only mentioned. The new generation Reploids are described as entirely immune to all viruses, including the Sigma Virus, thanks to their Copy Chips which give them complete control of their own bodies. When the new generation Reploids start their rebellion, X assumes it must be because they have Sigma's DNA in their databases, a theory which is later proved false.
Near the final battle, Layer asks X about Zero's relationship with Sigma, and he explains that Sigma has repeatedly tried to use Zero for his deep connection to the Maverick Virus. If Zero faces Sigma in the Sigma Palace, the latter thanks the former for introducing him to the Maverick Virus in the first place and allowing him to go Maverick.
By the time of the Zero era, the Sigma Virus has been deleted and is only mentioned in historical records. In Mega Man Zero 2, data on the computer at the New Resistance Base states the Sigma Virus was the cause of the Maverick Wars. In Mega Man Zero 3, data on the computer in Ciel's room further explains that X used the Mother Elf to bring an end to the Maverick Wars, indicating that the Sigma Virus was deleted some time prior to the Elf Wars.
- "Ultimate Justice on Parallel Earths", The Reploid Research Lavatory. March 2011.
- "There Were Originally 24 Maverick Hunter Units, and Other Mega Man X Tidbits (Updated)", The Mega Man Network. Feb. 12, 2018.
- "Does the Rockman Zero Collection Storyline Explain Everything?", The Mega Man Network. Apr. 9, 2010.
- Gallery, Mega Man X Legacy Collection. 2018.
- Kodansha Manga Hyakka #24: Rockman & Rockman X Himitsu Daihyakka. April 1995.
- Kodansha Manga Hyakka #34: Rockman X Chō Hyakka. April 1996.
- Game manual, Mega Man X3. 1995.