This article is from the X-Men Comic Books FAQ, by Kate the Short (firstname.lastname@example.org) with numerous contributions by others.
The Legacy Virus was a once-interesting plot device that served as a
parallel to the AIDS/HIV virus. Basically, it was spread from mutant to
mutant, and it caused mutants to die by accelerating or increasing their
powers to the point that they burned themselves out. Obviously, such a
nasty virus caused even more fear of mutants in the Marvel Universe.
As with all plot devices, it started out interesting and became a
cliche. The virus was supposed to be Stryfe's legacy--his post-death
gift to the world, if you will. Thus the name. However, one couldn't do
justice to the plot device by quickly creating a cure, since that would
not only cut the story short, but also trivialize the actual search for
an AIDS/HIV cure. So the story plugged away, year after year.
During its time, the Legacy Virus affected many mutants, including
Abyss, Bolt, Feral, Magik, Mastermind, Maverick, Moira MacTaggert,
Nicodemus, Omega Red, Pestilence, Psynapse, Pyro, Revanche, and of
course, Sinister's Assistant who let the darned thing out in the first
place. A team of mutants led by Exodus and Random and a set of
"Morlocks" created by the Dark Beast round out the medical list.
Most notable in the list above are Magik and Moira. Magik received the
virus after she'd been deaged post-Inferno, though a retcon during the
New Mutants mini-series revealed that teenage Illyana was infected when
Mikhail et al journeyed into the NM's past. Her death served as a huge
catalyst, since Colossus' grief led him to join the Acolytes. More on
that in a moment.
Back to Moira. Moira was a genetic researcher, of course, and so was
often exposed to nifty little things like viruses. The odd thing was
that she caught the virus AS A HUMAN. This was meant to parallel the
spread of the AIDS/HIV virus into the general population, but her
search for the virus cure waxed and waned as the plotline's prominence
varied due to shifts in writers and storylines.
Brian Fried reminds us that Zero give Douglock the key or cure to the
Legacy Virus a few years back. In "The Douglock Chronicles," which
introduced Douglock to Excalibur, Zero gave Douglock information to
cure the virus, and then was promptly destroyed for accessing that
information. (Side note: the issue was written by Scott Lobdell, who
also wrote the Excalibur issues in which Moira contracted the virus and
X-Men Prime, where the idea that the cure is connected to Cable's DNA
Every nasty Marvel Virus needs to have its cure, and one certainly was
around; the characters just didn't collectively realize it. Again, if
the virus cure was put off, lots more neat stories could be told that
used it as a plot device (or, as the case may be, neater stories could
be told that *didn't* use it as a plot device). The solution got placed
in a drawer until the writers and marketing staff had nothing better to
A second factor was Warren Ellis' apparent dislike for the storyline and
the switch from Moira's story to the Black Air / Pete Wisdom / Alien
conspiracy stuff that culminated in "The Black Air Trilogy" and
Excalibur #100. It wasn't until Ben Raab came on that the plot was
handled again. In a few issues, it seemed that Douglock would clue in to
the fact that he had the cure, and all would finally be resolved.
Marvel being Marvel, however, it was decided that the only reason people
were reading EXCALIBUR was for Kitty, Kurt, and Piotr; their moving back
to the core books would leave no purpose for Excalibur. Raab had to
close down his loose storylines to accomplish this. Of course, he also
couldn't reveal the cure for the Legacy Virus in Excalibur because those
types of events happened in the core books, like all other important
storylines (see the second WIZARD X-MEN SPECIAL for more info on this).
Raab left Douglock with a serious case of amnesia, in the hope that some
other writer could eventually use the material. Even the Dark Beast's
solution (in X-Men Unlimited #10) was completely forgotten so that Moira
and Mystique would have the honours of being involved.
In any case, Mystique and her terrorist brotherhood decided that the
best way to deal with anti-mutant humans was to mutate the Legacy Virus
so that it would infect the human population instead. She broke into
Muir Isle to get the information, and ended up blowing up Moira in the
process of destroying Moira's research lab. Moira, never one to give up,
fed the information from her research and Mystique's statements into
Xavier's brain, who finally passed it on to Legacy Virus researcher and
noted X-Man Beast.
Beast was quickly able to develop a cure; the problem was, he then
realized that since the original virus was spread once the first
infected mutant used his powers, the new cure would likewise only spread
once an infected mutant used his powers--thus sacrificing someone to
the greater scientific cause.
The FAQ will pause at this time to point out that it was pretty dumb of
Beast to leave the cure unlocked in a room with Piotr, who not only had
lost his sister to the disease, but also had lost his parents to murder,
his brother to we-can't-remember-where, and his former girlfriend and
teammate Kitty to they-never-told-us-but-she-came-back-afterward.
Piotr injected the cure, transformed into his organic steel form, and
passed away. This apparently released the cure into the air, so within
just a few days of Marvel Time the Legacy Virus was cured, Hallelujah!
This all happened in Uncanny #390. If nothing else good came of the
issue, at least Colossus can no longer be poorly written (as long as
Sinister remains far from his DNA, that is), and the plot device has
been resolved and buried.