"Another theory" said Father, "was that Homo sapiens evolved from the Neanderthals. That one was discredited and then revived several times. It turns out that was the closest theory to being right."

"You know, none of this explains why there are Neanderthals out here in the North American Wild Animal Park."

"You surprise me, Son," said my father. "I thought you would have leaped to at least some conclusion. Instead you seem to be passively awaiting our explanation."

I hated it when Father patronized me. He knew that, so he did it whenever he wanted to goad me into thinking. It always worked. I hated that, too.

"You brought me here because of the way I reacted to Elizio's death," I said. "And because you're famous scientists yourselves, you got to pull strings and get me a special tour. Not everybody sees this, right?"

"Actually, anybody can, but few want to," said Father.

"And the biohazard stuff-that suggests some kind of disease agent. What you said about the evolved-from-Neanderthals scenario being close to correct suggests ... there's some disease loose in the wild here that causes regular people to turn into cavemen?"

Father smiled wanly at Mother. "Smart boy," he said.

I looked at Mother. She was crying.

"Just tell me," I demanded. "No more guessing games."

Father sighed, put his arm around Mother, and began to talk It didn't take long to explain.

"The greatest breakthrough in the medical treatment of disease was the germ theory, but it took an astonishingly long time for doctors to realize that almost all human ailments were caused by infectious agents. A few were genetic-such as cystic fibrosis and sickle-cell anemia but those all seemed to be recessive genes that conferred a benefit when you had one of them, and killed you only if you had two. All the others--heart disease, dementia, schizophrenia, strokes, nontraumatic cerebral palsy, multiple sclerosis, most cancers, even some crimes-all were actually diseases.