"Now you're showing off, my love," said Mother.

"It was perfect," said Father. "And best of all, self-replicating yet nondestructive. Once you introduced it into a mother, it was in every egg in her body after a matter of days. Any child she bore would have this protection within it."

"It was perfect," said Mother. "The early tests showed that it not only prevented diseases, it cured all but the most advanced cases. It was the ultimate panacea."

"But they hadn't tested it for very long," said Father.

"There was enormous pressure," said Mother. "Not from outside, from inside the research community. When you have a cure for everything, how can you withhold it from the human race for ten years of longitudinal studies, while people die or have their lives wrecked by diseases that could be prevented with a simple inoculation?"

"It had side effects," I said, guessing the end.

"Technically, no," said Father. "It did exactly what it was supposed to do. It eradicated diseases with smaller-than-bacteria agents. Period. Nothing else. The only reason that they didn't immediately spread the counterinfection throughout the world to save as many lives as possible was because of the one foreseeable hitch. Can you think of it? It's obvious, really."

I thought. I wish I could say I came up with it quickly, but my parents were nothing if not patient. And I did come up with it after a few false tries, which I can't remember now. The correct answer: "Aging is a disease. You get this counterinfection, you don't die."

"We were concerned about a population explosion," said Mother. "Even if people completely stopped having children, we weren't sure that the existing ecosphere could sustain a population in which all the existing children grew up to be adults while none of the adults died off to make room for them. Imagine all the children entering the workforce, while the older generation, newly vigorous and extremely unlikely to die, refused to retire. It was a nightmare. So, by the mercy of God, the counterinfection was restricted to a large longitudinal study centered on Manhattan, a smallish college town in Kansas."