The Legacy Virus was derived by Stryfe, from a virus originally created by Apocalypse. Mr. Sinister was tricked into releasing it at the conclusion of the X-Cutioner's Song.
On opposite sides of the Atlantic, Dr. Moira MacTaggert, and Beast raced to find a cure for the deadly mutant plague. Before she was able to finish a cure, MacTaggert succumbed to a mutated strain. Beast, however was able to combine their work and reverse engineer a strain of the virus that would essentially eradicate all other strains. Once this harmless version was airborne, the cure would spread around the world in a matter of days. Unfortunately, the virus only becomes airborne upon the death of an infected mutant. Therefore, Beast's cure would require one mutant to sacrifice his life to release the cure. Believing this to be too high a price to pay, Beast abandoned the cure and returned to the drawing board. Colossus, however, never stopped mourning for his sister, Illyana Rasputin, who was among the first victims of the Legacy outbreak. Not wanting the plague to claim any more lives, Colossus snuck into Beast's lab and infected himself with the retro-virus, curing the world at the cost of his own life.
The New Strain
Recently, The Vanisher was hired by an unknown man to steal the Legacy Virus. He failed. Wolverine's team' X-Force, went to Mr Sinister's old lab to find and destroy the virus. In a battle with Cameron Hodge's group, The Right, X-23 was injected with the sample of The Legacy Virus. Elixir was able to "burn" it out of her system saying that it isn't the original, but it's worse in some ways. So far, Elixir is the only known cure for the new strain. The new strain now causes mutant abilities to flare out of control burning out the mutant host's body. The remaining Purifiers have been using this new strain to turn mutants in living bombs, detonating them in well populated areas so that the power flare kills the surrounding humans. Beautiful Dreamer was the first, Then Fever Pitch was used. Surge and Hellion were also to become bombs, but were saved through the efforts of X-Force.
Symptoms include purple lesions, coughing, fever, sweats, swollen glands, cardiac arrest, and uncontrolled mutant abilities.