The Legacy Virus first premiered in the wake of X-Cutioners Song, a story in the 90s that featured a crossover between the two main X-Men titles as well as X-Force and X-Factor. The Legacy Virus was created by Stryfe, a terrorist who was a clone of cable and a time traveller from 2000 years in the future (a future ruled by Apocalypse). The virus was released after Stryfes (apparent) death, a plague that infected mutants by targeting their x-factor gene. An infected mutant would eventually lose control over their powers, and Stryfes goal was that this would trigger a species war between humans and mutants. Inspired by the AIDS scare, the Legacy Virus was a background element in the X-Men comics for nearly a decade and claimed numerous victims, including Pyro, the original Mastermind, and Colossus sister, Illyana Rasputin/Magik. The idea of a plague that targets mutants fits right in with Apocalypses goals to ensure that only the strong survive. In the comics, Stryfe did base the virus on a similar virus created by Apocalypse in the future. And when the concept was loosely adapted in the 90s X-Men animated series, it was Apocalypse who created it.