Sigma Virus

The Sigma Virus (シグマウィルス), also officially referred to as the 'Suffering Virus' in Mega Man Zero Official Complete Works, is the commonly used form of Sigma after Mega Man X2, and the name is also used to refer to his version on the Maverick Virus from the Mega Man X series that turns Reploids into Mavericks.

Brief story
Not much is revealed about the different viruses in the series. Zero was the first Reploid to hold a "Maverick Virus", and it's commonly thought that Wily created this virus by analyzing the Evil Energy. However, Zero was not technically a "Maverick", as his evil personality was his natural state.

Several years after his creation, Zero is found by the Maverick Hunter Sigma, who fought against Zero and as a result had an unknown amount of the virus passed to him, merging with his DNA and later becoming the "Sigma Virus", with Sigma becoming the virus himself. After being repaired by the Maverick Hunters, Zero had no memories of his evil self and became good. Through, Zero was unaware that he still holds the original virus (possibly weakened after the fight against Sigma), and scattered it throughout the world during his missions as a Maverick Hunter.

In Mega Man X5, Zero notices he is immune to the Sigma Virus, which makes him stronger instead of affecting him. It is revealed that Sigma spread the Sigma Virus through the world to make Zero absorb a large quantity of it, intending to reactivate Zero's original self, now dormant inside him. It was also said by Alia that the Sigma Virus outbreak was affecting the humans, implying that the Sigma Virus could even infect biological organisms such as them.

Later, Gate was able to revive Sigma in Mega Man X6, but he was not complete and after awakening attacked Gate, saying he could revive without his help. In one of the endings from Mega Man X6, Zero acknowledges that he was the original carrier of the virus, and allowed himself to be sealed for over 100 years to be studied by a research facility and allowing to create a cure to the virus, leading to the end of the virus and the Maverick Wars years later, and the beginning of the Mega Man Zero series.

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. It doesn't have health for a final boss, and damage 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 similarly but smaller looking 3D wire Sigma attempts to possess X's body after his defeat, as the bodies he used where 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 through the world, and it appears as an enemy that looks like a purple Sigma head. If X gets hit with it several times, he will get a condition where he gets more damage from attacks for a while. If Zero gets it, he actually becomes stronger (mainly because he is the original bearer of the virus). It can only be destroyed by the player if the Virus Buster part is equipped, or by Zero's Z-Saber when using his secret armor.

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 final stages, a purple virus that looks like Zero. When it appears, it will try to infect the player. If it misses one time, it will reappear again and make another pass. The Zero Virus is two times more powerful than the Sigma Virus, though the results of being infected by it remain the same.

It's believed that the Zero Virus was created by Sigma to be like the original form of the Maverick Virus, the one that was carried by Zero before it infected him.

Trivia

 * In Mega Man X3, if Zero hadn't been disabled when you fight the Sigma Virus, instead of Dr. Doppler arriving to destroying the Sigma Virus, Zero will come and destroy the virus in his stead.