The Legacy Virus was introduced in the final moments of "X-Cutioner's Song," which pitted the X-Men against the time-displaced maniacal clone of Cable, Stryfe. Having "gifted" Mister Sinister with a canister that contained the disease, Stryfe's actions resulted in the virus being released into the Marvel Universe in 1993's X-Force #18, by Fabian Nicieza and Greg Capullo.
The disease quickly spread across the world, infecting mutants (and only mutants), and threatened the entire species until the sacrifice of Colossus resulted in a cure being spread throughout the planet. But before he made his fateful sacrifice, which notable Marvel heroes and villains actually succumbed to the disease?
ILLYANA RASPUTIN
Illyana Rasputin was the younger sister of the longtime X-Man, Colossus. After being trapped in Limbo for years, the then-teenaged Illyana joined the early roster of New Mutants. However, an encounter with a future version of her brother Mikhail resulted in Illyana contracting the disease, which didn't strike until after the events of Inferno restored Illyana to her original adolescent age. Despite the best efforts of the X-Men to save her, Illyana passed away in 1993's Uncanny X-Men #303 by Scott Lobdell and Richard Bennett -- prompting her brother to leave the X-Men and join Magneto's Acylotes for a time. Illyana would later be restored to life in the aftermath of House of M, eventually regaining her position with her friends to become a crucial member of the X-Men.
NICODEMUS & BURKE
Burke and Nicodemus are two of the Externals, a sub-species of long-lived mutants that are reborn following any death. Burke and Nicodemus also possess unique abilities: Nicodemus was capable of pyrokinesis, while Burke was gifted with precognition. Both were infected with the Legacy Virus, setting them on the path towards resurrection. Notably, Nicodemus lost control of his powers as he died, causing a major explosion in Fabian Nicieza and Greg Capullo's X-Force #20. Although both men would later be reborn, Burke was last seen inhabiting the body of an infant, removed from the conflicts that had long defined his ancient existence. Nicodemus instead returned to a form similar to his earlier life -- only to be one of the Externals sacrificed by Apocayplse to jumpstart the External Gate to Otherworld during the build-up to X of Swords.
THE MULTIPLE MAN
Jamie Madrox has been a longtime ally to the X-Men and served on multiple incarnations of X-Factor. It was during one of these periods when he went on a mission to the island nation of Genosha in X-Factor #91 by Scott Lobdell and Joe Quesada. Trying to save the life of someone infected with the Virus, one of Madrox's duplicates attempted to revive the mutant, and in the process contracted the disease. Madrox subconsciously remained split into two separate bodies -- with the infected one resuming his duties with X-Factor until X-Factor #100 by J.M. DeMatteis and Jan Duursema when Haven failed to save him. X-Factor buried the duplicate, unaware their friend was actually alive but briefly amnesiac by the trauma of his "death," until he was discovered by the team and restored.
MASTERMIND
Mastermind was one of the original members of the Brotherhood of Evil Mutants, capable of trapping others in illusions. He proved to be a particularly dangerous foe during the Dark Phoenix Saga. However, he eventually contracted the Legacy Virus. By the time of the Uncanny X-Men Annual #17 story "The Gift Goodbye," by Scott Lobdell and Jason Pearson, Mastermind had been rendered bed-ridden. Jean Grey, Iceman and Bishop went to see him and confirm his illness, only to be dragged into a twisted illusion by the dying man. However, Mastermind just wanted to make amends with Jean for his previous crimes, and upon hearing her forgive him freed her -- saving the trio before perishing. Mastermind recently returned to the Marvel Universe as one of the mutants revived by the Five on Krakoa.
PYRO
Like Mastermind, the fire-controlling Pyro was a prominent member of the Brotherhood of Evil Mutants -- albeit in a later incarnation, commanded by Mystique. Pyro learned he was infected with the Legacy Virus after he was prevented from coming to the Acolyte base of Avalon. Pyro went through a prolonged period where he dealt with the disease and the effect it had on him, occasionally working with the Brotherhood while also trying to renounce his more violent past. During the events of "Dream's End," Pyro was crucial in saving the life of Senator Robert Kelly from another assassination attempt. But the strain was too much of a toll on Pyro, resulting in the Legacy Virus finally claiming his life in 2000's Cable #87, by Robert Weinberg and Michael Ryan. Pyro would also be revived on Krakoa, where he's embraced a more heroic role as a member of Kate Pryde's Marauders.
INFECTIA
Infectia was capable of altering living beings on a molecular level. She became an enemy of the original incarnation of X-Factor, briefly reducing Beasts' intelligence and making him into a far more brutish figure. She became a somewhat recurring villain for a short time until she was infected with the Legacy Virus. Thanks to her powers, Infectia was able to stave off the effects of the Legacy Virus for a time, but eventually grew too weak and contacted the X-Men in X-Men #27 by Fabian Nicieza and Richard Bennett. Making peace with Beast and Iceman, the former brought Infectia to the roof so she could die while gazing at the sunrise. Infectia's death had a huge impact on Beast, leading him to dedicate himself to defeating the virus -- which he eventually did, albeit at the apparent cost of Collosus' life.
KWANNON
Although Kwannon began life as a Japanese assassin for the Hand, she died in the original body of the X-Men's Psylocke, Betsy Braddock. In one of Marvel's more infamous storylines, the minds of the two psychic mutants were swapped, with each losing parts of themselves and gaining parts of each other in the process. After taking the codename Revanche, Kwannon briefly joined the X-Men before revealing that she had contracted the Legacy Virus. As the virus began to affect her powers, she traveled back to Japan where she asked her former flame Matsu'o Tsurayaba to kill her so she could die on her own terms. After he granted her request, part of her psychic powers restored Psylocke's full memory upon her death. Since she was resurrected and returned to her original body, Kwannon has been an active participant in numerous black-ops missions for the X-Men's mutant nation Krakoa in Fallen Angels and Hellions.