The Mutant Virus is a virus that is responsible for activating the X-Gene in humans and giving them mutant powers, which leads to the creation of mutantkind.
After Captain America's disappearance toward the end of the war, some members of the US government relentlessly wanted to recreate the super soldier serum. They secretly hired a group of scientists known as "Weapon-X" in Canada to do so. They abducted a soldier known only as "Logan" because of his uniqueness; born with bone-like claws and a healing factor, as they thought he would be the key in recreating the serum. The scientists used Logan's DNA and accidentally created a new human gene, one which could one day turn mankind into extremely powerful beings, which inspired head scientist, Dr. Cornelius, who believed it to be humanity's greatest survival, and calls these altered humans "mutants." In a suicide attempt, Cornelius blows up the entire facility so the government won't get their hands on what he claimed to have created, and as a result, the mutant genome spreads as the Mutant Virus and would eventually take its toll on the planet; turning ordinary humans into mutants and eventually overpopulating the planet.