"The beauty of it, from the Kangas' viewpoint, wasn't just that it was lethal in so many different ways, but that they'd already been playing around with it for a couple of decades. They had its life cycle down pat and they'd been working on ways to aim it at specific DNA/RNA groups. That was what made it perfect for Midgard, because only one species on the planet used DNA at all: man. Actually, the biochemistry on Midgard isn't all that much different from Terra's, bearing in mind that we're talking two entirely different biospheres, but it uses a different complex of amino acids.

"So they revamped their parasite, accelerated its growth cycle, and dusted Midgard with it. Before we realized what they'd done, everyone on the planet was infected."

She looked out to sea, her face drawn, and Aston surrendered to a sudden impulse. He slid closer to her and reached one arm around her. Not really in an embrace, far less a caress, but simply to let her know he was there. She looked back at him and smiled, her eyes suspiciously bright.

"Anyway," she said in a voice which was just too calm, "it performed to specs. According to the records, it was incredibly painful, too, so perhaps it was merciful that it killed so quickly in most cases. The actual death rate was something like 99.8%. Out of just over two million people, there were exactly 5,757 survivors.

"But-" her eyes flashed suddenly, and he saw the she-tiger in her smile once more "-they'd expected a hundred-percent kill. They should've gotten one, too. The best theory is that their little horror was unstable and they got an unexpected mutation. Whatever, one tiny batch didn't kill everyone it infected. Most of them, yes, but not all. And in the case of those it didn't kill, it became not a parasite, but a symbiote. Not only that, it piggy-backed itself onto their chromosomes."

"Symbiote? Piggy-backed? I'm afraid I'm not with you yet, Milla," he said gently.

"It's simple, really." She turned to face him fully. "I mass about sixty-six kilos, but I tip the scales at just under sixty-eight. The other two kilos is my symbiote."

"That ... 'protoplasmic ooze' you mentioned?" he asked levelly.

"That's right. Only it's not as greedy as the original version." She smiled mirthlessly. "You might say it's a case of mutual advantage; it lives off my respiratory and digestive systems, and, in return, it protects its environment: me."

"Those wounds ..."

"Exactly. It used its own mass to seal the ruptured tissues while it kickstarted the 'regular' healing process. It even pulled me out of shock by tightening itself down around my arteries. It takes good care of me, because without me it dies."

"My God," he murmured, his voice touched not with disgust but with awe, and she responded with a more natural smile.

"I can't complain," she said. "It does some other nice things, too. It's infected my chromosomes. Effectively, I've got a couple of extra genes-dominants, I might add. And my symbiote's not a very gracious host; it eats anything-bacteria, viruses, whatever-that isn't tagged with 'our' genetic code. Which means, of course, that things like cancer and the common cold never bother me. On the other hand, even though I can eat just about anything in an emergency, my symbiote gives me fits over some things-like alcohol-and it also means that transurge would be all but impossible if I suffered catastrophic damage; unless they're cloned ahead of time, transplants don't carry the right genetic code, so they're rejected automatically. And if I'd been born with genetic birth defects, there wouldn't've been a damned thing that could be done for me-because the symbiote locks in the defect and won't let go. Even impacted wisdom teeth can be a real pain; they keep regenerating." She shrugged once more.

"On the other hand," she said softly, "it seems to regard old age the same way it does any other disease."

"You mean-?"

"I mean that every living organism eventually 'forgets' how to regenerate itself ... except people like me." She grinned crookedly. "That's one reason some Normals don't much care for us. Polite people pretend not to know it, but there're names for us. 'Thuselah' is the kindest-from 'Methuselah'-but the others are a lot nastier. It's easy enough to understand. The people who use those names get old and die; we don't. Why shouldn't they resent us?"

"But surely not everyone does," he said, and she shook her head.

"No. Some Normals see our women as brood mares," she said grimly. "We're not all that fertile-which is probably just as well, since our ova regenerate, too, and we stay fertile-but we tend towards multiple births, and all our children are born with the symbiote and pass it to all their children. For some reason we haven't quite figured out, we're just as fertile with 'normal' humans as with each other, so some male Normals see us as a way to beget 'immortal' children of their own." She brushed hair out of her eyes, and this time he understood the half-wry, half-bitter wisdom of the old eyes in her young face.

"Listen to me! You must be thinking we're some kind of persecuted minority! We aren't, really, but sometimes we feel a bit hunted and harried. Only about half the Midgard population is Thuselah, and the percentage is a lot lower everywhere else-there're less than a billion of us even now. The funny thing is how many of us feel most at home in the service. Maybe it's because the chance of dying by violence is so much higher there. I know there was a time in my life when I felt unspeakably guilty because I knew I would never get old-at least, not as long as my symbiote holds out. I suspect we're drawn to the military out of a need to share the mortality of the non-Thuselahs."

She gave the tiny toss of her head he was coming to realize was associated with the shifting of mental gears.

"The Navy and the Corps are glad to get us, especially in the interceptor squadrons. Fighters are a youngster's game, and our bodies and reflexes stay young while we go right on gathering experience. The casualty rate catches up with most of us in the end, however good we are, but that's fair. No one makes us hang on and hang on the way we do. We ... just do. It's almost addictive."

"I know," he said softly. She looked at him curiously, but he wasn't quite ready to talk about his own impending retirement from active duty ... or what those duties had been. "I've known a lot of fighter jocks in my time," he said instead. "The one thing they all dread is getting too old to strap on a fighter."

"That's the way it is," she agreed with a sigh. "Actually, it's even more addictive for a Thuselah, because we tend to be so good at it. We've got extraordinary reflexes-again, thanks to our symbiotes. Our neural impulses move about twenty percent faster than the norm, so we can get more out of a fighter. And when we have to, we can go a long time without sleep, because our symbiotes scavenge the fatigue products out of our blood. In a real emergency, they actually supply us with energy. It's a survival tactic for them; they keep us going so we can both survive. Until they exhaust their own stored energy, anyway. Then they start scavenging our tissues to keep themselves alive. When that happens, we're in trouble. We go into a coma and, without someone to feed us-" she gave him a warm smile "-our poor, stupid symbiote goes right on eating until it kills us both."

"My God," he said again, regarding her with so much wonder she actually blushed.

"Doesn't it ... bother you?" She sounded almost shy.

"Why should it?" he asked simply. "Oh, the idea will take some getting used to, and I'm not immune to envy, if that's what you mean, but I really don't think it bothers me." He gave her a smile of his own. "And you are human, you know-you're just the new, improved model. If I understand you right, this genetic modification is an acquired survival trait. Eventually, everybody will be like you."