Sigma Virus

The Sigma Virus (シグマウィルス) is Sigma's true form, a sentient virus first seen in Mega Man X2. It is also an alternate name to the analogous Maverick Virus that turns Reploids and Mechaniloids into Mavericks.

Characteristics
As a virus, the Sigma Virus has the ability to infect and corrupt its targets. 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. 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 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. Once infected, the Sigma Virus is relentless and the infected cannot be easily returned to normalcy. Only when the host itself has died does the Sigma Virus die along with it. Therefore, the only option for most infected Reploids is to be retired by the Maverick Hunters. 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. 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.

History
Surviving from over 100 years ago, an unknown computer virus emerged from the capsule Zero slept in. Driving robots mad locally, the human leadership acknowledged these infected robots as Mavericks, and ordered them to be disposed of. Following through with the disposal of Mavericks, the human leadership decided to have Mavericks exterminated by other Reploids and formed the Maverick Hunters. Ironically, the leader of the first generation of Maverick Hunters is Sigma, the Reploid who eventually led a revolt against the human leadership. Named after him, the virus would come to be called the "Sigma Virus".

After receiving information of Zero awakening from his capsule and causing violence, Sigma found and intercepted the newly discovered red Maverick. It was during this incident that circumstances led to both Sigma and Zero changing their allegiances. Zero had been infected with the unknown computer virus that had been leaking out from his capsule for quite some time, and during the battle, in the midst of experiencing possibly virus-induced effects, Zero was seriously damaged by Sigma. Due to this, Zero’s personality completely changed, and he would turn to work as a Maverick Hunter. At the same time, Sigma, also seriously damaged, was infected by the same virus coming from the capsule, and the virus was passed from Zero to Sigma.

After being showered with the virus, it began to germinate wickedness in Sigma's heart and somehow mutated within Sigma, merging with his DNA Soul. Several months later, Sigma was completely enslaved by the live and evil virus and his heart had become evil. The evil Sigma defected to the side of the Mavericks and used his overwhelming strength and genius intellect to subdue many Reploids into joining in a revolt against the human leadership led by him. This revolt began the wars between the Mavericks and the Maverick Hunters for the future of humanity; the Maverick Wars. After the destruction of his original body in Mega Man X, Sigma survived thanks to the virus, Sigma himself now being a sentient virus, which allows him to survive when his bodies are destroyed.

Mega Man X2
During Sigma's second revolt, the virus becomes publicly known as the "Maverick Virus" that Sigma uses to create more Mavericks to serve him. From the Computer Center, which is occupied by Magna Centipede and later Sigma himself, the Maverick Virus is sent throughout the world. At the Computer Center, it also appears in substantiated form as Chop Register. Sigma's viral form is also fought as the final boss, after his body is destroyed.

Mega Man X3
Sigma's extensive use of the Maverick Virus makes it the focus of Dr. Doppler's research. Dr. Doppler, aiming to create a world where humans and Reploids could coexist peacefully, determines that the Maverick Virus was the cause for the Maverick outbreak and he develops a revolutionary anti-virus. Using this anti-virus, it appears that Dr. Doppler is successful in suppressing any abnormal behavior in Reploids and preventing them from going berserk. As a result, many of the most advanced Reploids gather near Doppler as their new mentor and found Dopple Town, a perfect Utopian community.

With Dr. Doppler guiding them, the world seems ready to enter a new golden age, but a few months later the rapid growth of Dopple Town becomes its downfall as it is infected by the very virus it was believed to remedy. Dr. Doppler himself incited a new Maverick outbreak, causing chaos in Dopple Town and declaring war against humanity.

During this revolt, the Maverick Virus infects not only Dr. Doppler but also many others. Aside from its presence within the Reploids it infects, the Maverick Virus also once more appears in substantiated form as Shurikein; but the biggest impact it makes is near the end of the incident when Dr. Doppler made the startling revelation that Sigma's true form is that of a computer virus; indeed, Sigma is the Maverick Virus itself. Being one with the Maverick Virus gives Sigma the ability to bend Reploids under his will and to revive effortlessly after each defeat. It is because of this unique connection with Sigma that the Maverick Virus becomes known as the "Sigma Virus".

After X destroys Kaiser Sigma, Sigma's latest body, in Dr. Doppler's Laboratory, Sigma himself finally substantiates in his viral form. As he was not able to defeat X even with a powerful body, Sigma decided to possess X's body.

Unlike Mega Man X2, the Sigma Virus is invincible. The player must Wall Kick up the walls, making sure the Sigma Virus doesn't knock X into the rising lava. After reaching the top, X is cornered, but help arrived in the form of Dr. Doppler's anti-virus. Initially, when Dr. Doppler himself was revealed to be the mastermind behind the rebellion, his anti-virus was considered a placebo. However, Dr. Doppler did indeed develop a working anti-virus, attracting Sigma's attention and in turn causing himself to be infected. This anti-virus is used in the conclusion of the game in an effort to defeat Sigma once and for all. However, because X3 has two different ending scenarios, it is possible for either Doppler, who sacrificed his life or Zero who had equipped the anti-virus program onto his Z-Saber, to deliver the final blow to Sigma. Struck by Doppler's anti-virus, Sigma feels his own program fading, but even that does not stop him and his virus from recovering and appearing in Mega Man X4 and beyond.

Mega Man X5
The Earth Crisis incident sees the first large scale use of the Sigma Virus. In an attempt to awaken Zero to his true self, on his elderly partner's advice, Sigma spreads the Virus all over Earth. This plan causes worldwide chaos as Reploids all over turn into Mavericks without warning. Even the Maverick Hunters themselves aren't spared from its devastating effects. This wide spread use of Sigma Virus, in combination with the fall of Eurasia, makes the Earth Crisis incident one of the most cataclysmic events of the Maverick Wars.

According to Sigma's plan, Zero himself could not escape infection. However, as Zero is the original carrier of the Maverick Virus, it does not affect him negatively. As a matter of fact, Sigma explained that whenever the Sigma Virus infects Zero, his power output is actually growing because the virus is purifying his body. Signas mistakenly believes this to be because Zero's body might have anti-bodies for the virus. Lifesaver, on the other hand, disagrees with his superior's theory and wholeheartedly believes Zero to be dangerous and that his reaction to the virus should be monitored closely. Though close to the truth, Lifesaver's fears prove to be unfounded; Zero simply refuses to awaken to his true self. Sigma quickly learns that this was because the amount of Sigma Virus is simply insufficient to awaken him. With this new information, Sigma takes his plan to the next level by turning rundown Eurasia into a virus colony. The virus spread on board Eurasia would combine with his Sigma Virus at the point of impact and give birth to a new virus; the Zero Virus. This Zero Virus, that closely resembled Zero himself, has the potential to awaken Zero that the Sigma Virus lacked.

At the conclusion of the incident, Sigma and his Sigma Virus fight with X and Zero to the death. As a matter of fact, Zero himself believed that he had finally defeated the Sigma Virus once and for all, a notion not far from the truth. Zero does indeed succeed in completely killing Sigma, but the Sigma Virus threatening the world still remained at large.

Mega Man X6
Shortly following the Eurasia incident, Gate inspects the crash site (Area Zero) and uncovers a piece of Zero's body. This piece of Zero has a peculiar aura because it is actively spreading the Sigma Virus. Gate himself becomes infected by the Sigma Virus and is driven insane. Despite being driven insane, he succeeds in analyzing a large portion of Zero's DNA and perhaps even noting the connection between Zero, the Sigma Virus, and Sigma. Using his understanding, he creates a new subspecies of Sigma Virus (The Nightmare) and somehow succeeds in accidentally starting Sigma's revival and bringing him back. Though Gate plans to use Sigma to further his own plans, Sigma himself does not obey and betrays the one that brought him back to life, even altogether disagreeing that he needed Gate's help, to begin with. In the end, Sigma was destroyed by X and Zero, but once again relying on his Sigma Virus, he managed to return and cause another incident.

Mega Man X7
After being nearly hunted down by Red Alert, the crime boss Cedar acquires a "Sigma Virus Mk.2" as a means to guarantee his own safety. Red Alert, however, deems this type of protection useless and decides to send Axl, who is immune to the Sigma Virus, on a mission to eliminate Cedar. Though Axl succeeds, the success of this mission attracted Sigma to Red Alert. Under the guise of "the professor", Sigma uses his Sigma Virus to manipulate Red Alert, going so far as completely controlling its members in order to force its leader Red into cooperating with him in exchange for their freedom.

Mega Man X8
The Jakob Project sees the introduction of the New Generation Reploids, who were all equipped with the same copy ability as Axl. These Reploids, of which Axl is the prototype, advertise their complete viral immunity as a reason for why they cannot go Maverick. As such, when the New Generation Reploids rebel against humanity, it is proven beyond any doubt that this time around, the virus is not responsible for their Maverick behavior.

Eventually, it is revealed that this new Maverick behavior is instead caused by Sigma's DNA data, which was included in their copy chips at the time of their creation. This DNA data causes the New Generation to become like Sigma himself. Lumine, the leader of the New Generation Reploids, elaborates on this; according to him, Sigma was not insane when he rebelled against humanity, and in fact, his and their rebellion are both of their own free wills.

In this incident, Sigma's programming seems to have been completely destroyed by X, Zero, and Axl. However, just as before, the Maverick Wars continue and the Sigma Virus remains a constant threat.

Future
As a result of the Nightmare Incident that Gate started, Zero is discovered as the source of the Sigma Virus and the Nightmare; it is proven that Zero was a carrier of the Sigma Virus, and was contagious to his surroundings as he worked across the world as a Hunter. In finding this, the human leadership decides for Zero to be sealed away and Zero by his own volition admits himself into a research institute fearing he might become a disturbing presence. Intending to sleep for 102 years, Zero is subjected to 50 years of Sigma Virus related experiments. Due to the influence of scientists studying the Sigma Virus, his cognitive program is removed from his body so the two could be researched separately.

A Reploid researcher of this time (Dr. Ciel’s great-grandmother) studies Zero’s cognitive program and discovers an antibody to the Sigma Virus. She completed the Sigma Anti-body Program called Mother Elf, a program lifeform that can rewrite and erase the Sigma Virus. Using her, X starts turning Mavericks back to normalcy, completely eliminating the Maverick threat by bringing the number of Mavericks to an all-time low and so effectively ending the Maverick Wars as well as erasing Sigma from existence for good. However, near the end of the Maverick Wars, the misuse of the sentient computer programs called Cyber-elves and the Mother Elf being cursed by Dr. Weil, would mark the beginning of the worst war recorded in history, the Elf Wars.

Mega Man X2
In Mega Man X2, the Sigma Virus is the final boss, a 3D wire frame head of Sigma that can only be harmed by a charged shot, Strike Chain or Shoryuken. Damage done to it can only be determined by the color of its appearance (green for default, red for critical). It attacks by firing lasers from his eyes or mouth and making two enemies appear randomly from his mouth, the enemies being Tubamail-S, Tiranos, Scriver and Fishern. After losing some health, he will also warp around X's current location to hold him inside his frame, and when successful will cause damage to him until he escapes.

Mega Man X3
In Mega Man X3, a similar but smaller looking 3D wireframe Sigma attempts to possess X's body after his defeat, as the bodies he used were not strong enough to defeat X. However, the vaccine created by Dr. Doppler weakened him and X was able to escape. Sigma was almost deleted by the vaccine program.

Mega Man X5
In Mega Man X5, Sigma spreads the Sigma Virus throughout the world, and it appears as an enemy that looks like a purple Sigma head and is present in half of the eight Maverick stages. If a character gets hit by it four times (or eight times if they are equipped with the Anti-Virus Guard part), they will become infected and will have a certain condition: X will gradually lose health for a short period of time, which can ultimately kill him if his life bar gets fully depleted, while Zero, on the other hand, will regain all lost health (mainly because he is the original bearer of the Maverick Virus) and become temporarily invincible. The Sigma Virus can only be destroyed by the player if the Virus Buster/Shot Eraser parts are equipped by X and Zero respectively, or by Zero's Z-Saber when using his secret armor.
 * If the "VIRUS" state has been activated, X can't perform Giga Attacks during the virus-infected state period.

Other appearances
During the events of Marvel vs. Capcom: Infinite, Ultron Sigma managed to modify the Sigma Virus to affect living beings, infecting several Asgadians, including Thor Odinson and a Dah'ren Mohran. Under associate with Ultron Sigma, Grandmaster Meio is tasked to develop Gravitron Core which houses the Sigma virus and enhanced the virus with the portion of the Power Stone, using the core as an air strike gas bomb to spread the virus across the globe. Though the heroes manage to destroy the symbiote with three Infinity Stones they have obtained, they failed to stop the Sigma virus spreading. During the heroes' final raid on Xgard to save themselves and Earth being completely infected by Sigma viruses, Chris Redfield's infection began to process, but he manage to negate it with his willpower. It is revealed that Ultron and Sigma virus' weakness is the Infinity Soul Stone, curing those who are infected by the virus permanently, and the said stone true potential only works for those who have a noble soul, something Ultron and Sigma do not have, as mentioned by Thanos, Jedah Dohma and Dante. The Soul Stone also not only cause Ultron Sigma went more insane and feels painful, it also transformed the fused machine into Ultron Omega.

Trivia

 * Originally "Maverick Virus" was the term used to describe the computer virus that turns Reploids into Mavericks. Since the events of X3, it is more commonly referred to as the "Sigma Virus" instead of the "Maverick Virus". From X2 to X4, resources alternate between "Sigma Virus", "Maverick Virus", or describing it as a "Maverick-facilitating virus", a "virus that facilitates Maverick behavior", or a "Maverick converting virus". From X5 onwards, resources began to use "Sigma Virus" as the only name by which the virus is known, regardless of it being inappropriate in reference to the time of before Sigma's infection; since then the term "Maverick Virus" is seldom, if ever used. With the introduction of new viruses as the series progressed, the term might perhaps be used as a generic term to encompass a wide variety of comparable computer viruses (see Zero Virus, Nightmare).
 * Rockman Perfect Memories notes that it is said that Zero was born with the Sigma Virus inside of him. The book postulates that it might have been Zero's creator Dr. Albert W. Wily who had put the Sigma Virus inside of him.
 * However, as the Rockman Zero Collection Timeline reveals, Zero wasn't born with the Sigma Virus inside of him; instead, it was spread by Zero's capsule in order to remove the cognitive flaw in Zero's head which made Zero violent and disobedient.
 * Similarly, Bass's ending in Mega Man 10 also hinted at the Maverick Virus and Roboenza being the "other project" that Wily alluded to in addition to creating his ultimate creation.
 * Gate from X6 once dreamed of creating a cure for Sigma Virus. However, circumstances ironically led him to be infected by the same virus he tried to cure. Gate's dream was inherited by Alia, but it is unknown if she was involved with the creation of the Mother Elf.
 * At first glance, it appeared that Lumine contradicted the long established fact of Sigma's change of allegiance thanks to the virus, but in a conversation between Zero and Sigma, this is alluded to; Sigma made mention of the virus from Zero's DNA that had entered his systems. Sigma emphasizes that since that time, the seed for the destruction of the world had been planted. With Sigma giving Zero credit for the current state of affairs and Lumine's assessment of Sigma's sanity, the truth about Sigma's rebellion seems to be somewhere in the middle; though the Sigma Virus turned Sigma evil, it never turned Sigma insane. Different from a person that had gone insane, the evil Sigma relied on his logical thinking to come up with his philosophy of evolution, which the New Generation share with him. However, the influence of the Sigma Virus' change to Sigma can never be forgotten. After all, Sigma's philosophy of evolution, based on survival of the fittest which justifies sacrifices as necessary, requires a distinct lack of compassion by those that follow it. As such, Mega Man X has always stood as a polar opposite to Sigma and his New Generation Reploids.
 * The Sigma Virus gets a brief mention in Mega Man X: Command Mission in Marino's concept art. According to her concept, she was intended to steal technology because she was searching for cures for modern robot illnesses such as being infected by the Sigma Virus.
 * The Rockman Zero official site once detailed the relationship with the Sigma Virus and Cyber Elves. This text has however since been removed the site. The site detailed that after Zero seals himself away, many scientists continued to study the Reploid mind for answers, and it became generally acknowledged as not a program itself but the "volition of a bodiless life form." In the field of research focused on the activity of the Sigma Virus on the Reploid thought program, it was proven that the virus possessed an effect of "volition." That "volition" became known as a Cyber Elf, and additionally any such volition that had the ability to influence things in a similar manner (Sigma Virus included) became designated Cyber Elves. Afterward, the scientists in the field of researching the Sigma Virus make researching Cyber Elves their focus. The study resulted in reaching the level where it became possible to strengthen a Reploid by means of a Cyber Elf.
 * The virus from space that was once mentioned on the Rockman Zero Collection official site shares many similarities with Roboenza and the Alien Energy, seemingly describing either one of those. As such, it is entirely possible that the Sigma Virus is indeed a subspecies of either the Roboenza, the Alien Energy, or both.
 * The Rockman Zero Collection official site also once said that the source of the first Maverick incidents was found to be the Sigma Virus emerging from Zero's capsule. However, the existence of the Sigma Virus was not known until the second revolt, and its link to Zero was not discovered until after the Nightmare Incident. As the relation to Zero does not necessarily make the connection between the first incidents, the virus, and his capsule, when this fact was discovered is not entirely made clear. At the very least, it is known by the time of the ZERO series.
 * In Mega Man Zero Official Complete Works, the Sigma Virus is the subject of the section "The three keys to solving the mystery of Zero and Omega" as the first of the titular "keys". The purpose of this section is to examine the background information regarding the three key elements that tie together Zero and Omega in an attempt to reveal some previously unknown angles to the story. The beginning of this section however clearly notes that the information that follows comes from the early concept phases for the Zero series' scenario, and therefore may not be canonical. The primary purpose of the first key "Sigma Virus" is to elaborate on the relation between the Sigma Virus and Zero by seemingly reciting what was implicated in X4 before further elaborating on Zero's motivations in his ending from X6. However, the section is also cited as containing very controversial information by people that choose to ignore the disclaimer that precede it; the first key especially is said to change many well known details about the Sigma Virus and its origins. This is because of its opening paragraph where it is briefly mentioned that the true essence of Sigma Virus is a Suffering Circuit Dr. Light inserted into X during the time of his creation. Whilst the text does explain the function of this circuit, it never goes into the exact details of this relation and leaves it as a vague allusion. It is exactly because of this vagueness, and the preceding non-canon disclaimer, that any interpretation of this section that is in conflict with established fact from the X series, should be disregarded.
 * From that same section, the second key "Forgotten Laboratory" elaborates on the Sigma Virus related experiments carried out on Zero. In that same laboratory two researchers, Dr. Weil and a distant female ancestor of Ciel's, researched tirelessly to end the war. By using properties and samples of the Sigma Virus, she created the Mother Elf, who had the ability to turn Mavericks back to normalcy. Indeed she was able to reverse the effects of the very virus that was used to create her. The section further elaborates that the Mother Elf was eventually overwhelmed by the effects of the Sigma Virus used to make her, which caused her to become Dark Elf and setting the Elf Wars into motion. However, the interview section in Mega Man Zero Official Complete Works tells a slightly different story: Originally, it was understood that Mother Elf was created using some properties of the Sigma Virus as a Cyber-elf with the ability to control Reploids, but some of the negative properties of the Sigma Virus were also transferred during the process, resulting in Dark Elf. As Inti Creates ended up changing a couple of those details and the information from "three keys" is also part of the early ZERO-series scenario, the factual relation between the Mother Elf and the Sigma Virus remains unclear.
 * The same section's third key "Elf Wars", notes that Dr. Weil always had his eye on Zero's body because of his body having a natural immunity to the Sigma Virus. He knew that a Reploid with no risk of going Maverick would make the perfect weapon to bring an end to the war. To that end, he designed Omega and created him from Zero's body. Do note, however, that like the other two keys, the same disclaimer on canon still applies.
 * In Mega Man X3, if Zero has not been incapacitated, he will arrive and destroy the virus with his Z-Saber, augmented with a vaccine made by Dr. Doppler. If Zero has been incapacitated, Dr. Doppler will instead take the virus into his own body, which houses the vaccine, and self-destruct.
 * The Sigma Virus in Mega Man X2 is one of the few final boss forms that doesn't have a visible health bar throughout the fight, the other being Master Albert's throne in Mega Man ZX Advent.
 * The Sigma Virus and its role in turning Sigma maverick was given an indirect reference in the Vile Mode for Mega Man Maverick Hunter X, where Sigma when freeing Vile in the hopes of recruiting him specifically stated that he needed someone who could "go Maverick on their own accord" (implying that Sigma's going Maverick in the game and the OVA The Day of Sigma was not of his own choosing).