It's thousands of years ago for them, but probably not all that many years in our future. All that's left of the human race, so far as we know, is one primitive ark-ship that arrived on the (fortunately empty) planet of Cassus. The human survivors of the exodus unpacked their ship, and set about building cities, and colonizing the planet.
And then one late October night, a guy whose name is remembered (probably wrongly?) as "Jack Shade" staggers into a hospital and dies. In less than an hour, everybody in the hospital is dead. Not much longer than that, and everybody who left the hospital during that hour, staff or visitors or discharged patients, is dead. Within an hour after that, everybody they've come in contact with is dead.
So workers in moonsuits start quarantining the known infected, and burning the bodies in increasingly huge pits, and it does no good. The moonsuits don't protect them. And people who didn't know that they were already infected have fled to every human settlement on Cassus.
They try everything. Nothing works. There is no vaccine. There is no cure. And there's no way they're going to find one in time.
And in the middle of this crisis, one little girl arrives at a hospital, obviously sick with Shade's Plague. They put her in with the rest of the dying, to be picked up after she dies and burned with the rest. But an hour or so later, she walks out under her own power. Her immune system has evolved a defense. For the first time there's hope. Except ...
Except that everybody who could possibly work on culturing her antibodies into a cure is already sick, with minutes left to live. So they try direct transfusions from her bloodstream ... and it works. As more scientists and biomedical engineers show up, called in from the now-burning cities around, themselves sick, she offers more transfusions. And more transfusions.
And finally she volunteers that it doesn't make any sense to save one person, herself, when there are so many more who can be saved. She volunteers to be drained of the last of her blood, to save as many people as possible who can't wait for the cure to be manufactured. Sobbing, they kill her for the cure.
And as they dispense the cure to the last however many human survivors in the morning, it occurs to them that they don't even know her name. But they sing about her every late October, the Angel of Cassus, the little girl who gave her life that the human race might live. And when that myth met the psionic amplifiers of Nexus ...
"Jack Shade's coming, sure as Death, / To lay you down, and steal your breath. / Put on your mask, and hide with care, / And pray the Angel finds you there. / Salvation's near. We'll carry on. / But first? Survive, until the dawn."
