Zero Virus

The Zero Virus (ゼロウィルス) is a powerful virus created by Sigma by combining the Sigma Virus and the virus variety from the space colony Eurasia (usually referred to as the "Colony Virus") in Mega Man X5. It appears as an enemy in the Zero Space stages, as a purple virus that looks like Zero.

Characteristics
The Zero Virus appears as a fuzzy, purple image of Zero in his original appearance from Mega Man X. It is much faster and more dangerous than the Sigma Virus, though the results of being infected by it remain the same. It also has the power to materialize the Cyber Space and programs such as TriScan Gold. The Zero Virus is not to be confused with the virus that Zero was originally created with.

In the alternate scenario from Mega Man X5 in which the colony crashes into the Earth, the effects of the Zero Virus on Zero's mind and body are made clear. Not only does Zero gain a significant increase in battle capabilities, but the virus also causes Zero's true self to shine through; he feels pleasure from the infection and a clarity about his true nature and purpose. As a consequence, he turns into a cold and emotionless robot who no longer cares about the Mavericks, and only wants to destroy X.

The Zero Virus materializes in certain points of Rei Space, floating for a few seconds before dashing towards the player to infect through contact. If it misses one time, it will reappear again and make another pass. Zero Virus infection causes the Virus Gauge to rise two levels instead of one from the Sigma Virus.

History
The Zero Virus is created by Sigma, who resolves to find out what Zero truly is after conducting thorough research. Sigma meets an old man (believed to be Dr. Albert W. Wily) who tells an interesting story about Zero's true nature, and on his advice, Sigma scatters the Sigma Virus all over the Earth so that it could infect Zero and awaken his true self. However, the amount is still insufficient, so Sigma decides to turn the space colony Eurasia into a virus colony before bringing it down to the Earth. After the Maverick Hunters' Space Shuttle operation, the Zero Virus is born from a reaction that combined the Sigma Virus from earth with the scraps of the virus from the colony. The name "Zero Virus" is coined by Alia, who notices similar readings between the virus and Zero himself.

However, even the Zero Virus fails to restore Zero's true self and Sigma is eventually defeated in the depths of Zero Space. Zero dies in battle, while X returns to the Hunters after being saved by an unidentified entity. The Zero Virus is not seen again since.

Alternate history
After Eurasia crashes, Zero is infected with massive amounts of the Zero Virus, causing him to hear the voice of his creator inside of his head. The voice tells him to wake up, assuring Zero that he is the best of all his works and ordering Zero to destroy "it". Zero reveals his true objective by telling X that he will destroy him. They confront each other, and Zero reveals that despite being infected with the virus, there is no virus reading, only Zero's normal data. Zero further elaborates that his mind is becoming very clear, as if he had known everything. He notes that he had stopped caring about the Mavericks and that there was just one thing that needed to be done. In response, X claims that he senses a greater evil than Sigma within Zero and resolves to destroy it.

In the aftermath of that battle, Zero reverts in order to rescue X from a sneak attack by Sigma, dying in the process. Sigma decides to retreat, making it clear that Zero almost returned to his true self, but much to his dismay, Zero did not quite reach it despite the potential he held.

Trivia

 * The Zero Virus is referenced in a conversation between X and Layer in Mega Man X8. X informs Layer, who is worried about Zero's well-being, that Zero was once used by Sigma to create a virus, and that Zero is upset that his data was used in attempt to destroy the world. Although X thinks that Zero should not blame himself and that the problem was solved anyway, Zero frequently expresses a grudge against Sigma for his past attempts to use him.
 * Where the Sigma Virus takes on the appearance of Sigma, the Zero Virus instead appears as Zero. However, as Zero underwent a design change in Mega Man X2, the Zero Virus appears to alternate between Zero's design from Mega Man X and Mega Man X2 in its animation.
 * If the "awakened" Zero is not defeated within two minutes in the alternate scenario, he will begin to use the Genmurei (an one-hit kill technique) relentlessly and his body will become invulnerable to all forms of damage. This event might symbolize his full awakening through the Zero Virus.
 * Within the fandom, there are many claims from misinformed fans, which state that the Zero Virus already existed within Zero, but was downgraded when it merged with Sigma.