WHAT THE...: At the end of the "X-Cutioners song" story line(Funny enough, the X-Cutioner never made an appearance...), Stryfe unleashes the something he calls his "Legacy", who's effect is unknown.
SYNOPSIS: Fast forward a few issues. It turns out "Legacy" is a mutant-specific disease, which at first causes weakening, then unsightly blisters appear, then, just before death, the mutants' power(s) reach thier maximum potential and explode, taking the mutant with it.
Illianya, Colosus's sister, was the first to fall. This lead to his defection to the acolytes in "Fatal Attractions". The Psylocke/Renevanch deal was set to close when Renevanch died, creating a massive explosion throughout the astral plane. Threnody, a mutant with the specific ability to detect mutants with the disease, was introduced, just as the ex-villianess Infectia was put to death. It has been hinted that other, unknown mutants have died of the virus, too. Jamie Maddrox, the Multiple Man, has the virus and so does Mikhail, Colosus's and Illiana brother. There's a funny story behind that one (not funny as in ha ha but as in ironic). Y'see, Illiana used to have all sorts of cool magic powers and stuff, so Mikhail went back in time to get her magic-filled blood so he could cure himself. However, not only did that not work, but he infected Illiana as well. Oops. Another casualty was Mastermind, a Member of the Brotherhood of Evil Mutants I and the Hellfire club I.
However, now it seems Dr. McTaggart has the virus. This indicates that either the virus is NOT mutant specific, or that the Doctor herself is a mutant. A third suggestion that has popped up is her "various" and "intimate" contacts with mutant, Proffesor X and Banshee to be more precise, has made her more subjectable. A forth idea is that the disease is PERSON specific, and since there would be no way to prove it, it would be the best way for Stryfe to of administered it. The only other mutant with the virus that I know of is Pyro. There have been hints that Mr.Sinister has the disease as well. Some others include one of Multiple Mans clones (who died, everyone thinking it was really him), and Avalanche (I think).
The aformentioned Sinister has been working long and hard on the disease, taking very drastic steps. Beast thinks he's close to a cure, and Cable may have a sub-consience recolection of the cure. But until we're given definate answers, we won't know for sure about anything, especialy since the virus storyline has taken a backseat lately.