In the fictional Marvel Universe, Legacy was a devastating plague that ripped through the mutant population, killing hundreds, and mutating so that it affected baseline humans as well, until it was cured almost overnight by the selfless sacrifice of the great hero, Colossus (comics).
Description
Legacy is an airborne viroid that only affects humans with an active X-factor in their genetic code (commonly referred to as mutants). The viroid prevents the body from producing healthy cells, acting like a fast cancer, and causes the victim to lose control of his mutant powers until he "burns out" and dies.
The viroid mutated, creating a strain that attacked all humans, and not just mutants. The only known Legacy-infected human was Moira MacTaggert, but she was killed by Mystique before the disease could kill her.
History
During the X-Cutioner's Song crossover, the villain Stryfe gave Mister Sinister a canister that he claimed contained two thousand years' worth of genetic material from the Summers bloodline. When Sinister opened it after Stryfe was apparently killed by Cable, he found nothing inside. Far worse than that, the canister actually contained a plague, Stryfe's "Legacy" to the world.
When Colossus's sister Illyana fell ill and died, he left the X-Men and joined Magneto's Acolytes, but later returned. His teammate Beast developed a cure for the virus, but in order for it to be quickly dispersed, a mutant had to sacrifice himself. Colossus did not want any more to die like Illyana had, so he injected himself with the Legacy cure and transformed into organic steel.
The X-Men found him lying on the floor, dead, but Legacy had been stopped.
Unfortunately, this rapid cure had unforseen geo-political effects. Thousands of Legacy-infected mutants and mutates had been quarantined on the island nation of Genosha, ruled by Magneto at the time. The instant cure gave Magneto a vast army overnight, and he plotted world conquest in the "Eve of Destruction" crossover.