Before Megaman 2, Dr. Wily started working on a robot that could input destructive computer viruses, change a machine's allegiance, heal broken down machines, and steal any machine's powers. However, as Dr. Wily was putting in the robot's bonding chip, Mega Man attacked and destroyed his castle. The mad doctor was torn between his prized robot and life, and he left, but not before leaving the unfinished robot with a tech machine. For years until after Megaman 10, the virus robot stayed in a liquid state. Then, as he woke up in the rubble of metal and brick, the robot grabbed the tech robot and sucked the power out of him, gaining the ability to fix things himself. Finally, after a year, he was finished, with upgrades beyond imagining.
He walked away from the destroyed castle and knew that whoever did this was powerful and, therefore, worthy of his power. He traced the scans of the buster scorch marks and traced it to a robot named Rock. Setting off to prove himself the strongest, he sets off towards the last trace of Rock. After days of walking, he meets Bass in a valley, who had enough data to match the robot Rock. After a long battle, Bass loses his left arm and faints, but not before asking who the robot was. Because he was never classified with a name, he takes it upon himself to name himself the robot's fear: Virus. He copies Bass's rapid fire and Fusion Ability, before blowing up the valley with Nuclear Smash.
He moved on quickly to storm his creator's nemesis's lab: Dr Light's Laboratory. Light sent out all Megaman 1 and Megaman 2 robot masters to combat him. After narrowly defeating all of them, he copies their abilities and moves on to the next room, where Megaman, Protoman, and Dr. Light are just finishing making the ultimate robot killing machine. They unleash E9-1122: Warp Man. Able to teleport to different parts of the room and armed with twin arm cannons, Virus and Warp Man face off. In the end, Virus wins, although his power is running completely low. Warp Man, using his last bit of energy, opens a warp hole, sending Virus spiraling into it. He is assumed dead in the Megaman Universe, but sometimes Megaman wonders.......
If you want to see what ACTUALLY happened to Virus, go to the Sonic Fanon Wiki and look up Virus EX!