He paused and took a breath. “I want to tell you about something. Let me talk to you about symbiosis.”

Imagine a virus… a simple bundle of nucleic acid packaged inside a protein shell… a killer, a smart bomb with only one job—replication.

Suppose this virus finds a vector, and penetrates the skin and outer membranes of a multicelled organism… perhaps a human being At that point, its job has only begun. From there it seeks its real prey, not the man so much as a single one of his trillion cells.

Seeking might not be the proper word. For a virus is only a pseudo-lifeform. It doesn’t propel itself after vibrations or chemical traces, as protists and bacteria do. A virus only drifts, suspended in water or blood or lymph or mucus—until it strikes the surface of an unlucky cell.

Now suppose one of these little bits of half-life is lucky. It has evaded the victim organism’s defenses. No antibodies manage to latch on to it and carry it away. It isn’t engulfed and destroyed by the immune system’s strike forces. The fortunate virus survives to bump against a likely cell in just the right way, triggering adherence.

It sticks to the cell wall, a simple capsule of protein, ready to inject its contents into the prostrate prey. Once inside, the viral RNA will take over the vast, complex chemical machinery of the cell, forcing it to forge hundreds, thousands of duplicates of the original virus, until, like an overstretched balloon, the ravaged cell bursts. The new viral horde spills forth, leaving only wreckage behind.

There is the virus, stuck to the outer wall… poised to inject this tyrannical cargo into the prostrate prey…

Prostrate, yes. But helpless?

For a long time an argument raged among physicians, biologists, and philosophers. A small minority kept asking the same question over and over again.

“Why does the cell let this catastrophe happen?”

Biological heretics pointed out how difficult it was to seize and penetrate the intricate barriers of a cell wall. So much was involved, and it would seem so simple for a cell merely to refuse access.

What about the fantastic number of steps needed to turn the machinery of the cell into a slave factory, forcing the ribosomes and mitochondria to perform tasks totally alien to their normal functions?

“All the cell needs to do is interrupt any one of these steps, and the process is stopped, cold!” the unbelievers declared. “There must be a reason. Why does the cell allow itself to be such easy prey?”

Classical biologists sniffed in disgust. Animals develop new ways to fight viruses all the time, they said. But viruses evolve methods around every obstacle. The balance is always struck across a knife edge of death.

But the dissenters insisted, “Death is nothing but a side effect. Disease is not a war between species. More often, it is a case of failed negotiation.”

“You’re losing me,” Carl told Saul.

Saul drummed his fingers on the desktop and searched for the right words. “Hmmm. Let’s try an example. You know what mitochondria are, right?”

Carl inclined his head and spoke in a hollow voice. “They’re organelles… internal parts of living cells. They regulate the basic energy economy… take electro-chemical potential from burning sugars and convert it into useful forms.”

“Very good.” Saul nodded, impressed. Carl had, indeed, been studying over the long, hopeless years. No scholar, he had probably mastered the material by brute force. “And you know the widely held theory over where the mitochondria came from?”

Carl closed his eyes. “I remember reading something about that. They resemble certain types of free-living bacteria, don’t they?”

“Yes, that’s right.”

“Some people think they were once independent creatures. But long ago one of their ancestors got trapped inside one of the first eukaryotes.”

Saul nodded. “About a billon years ago… when our ancestors were only single cells, hunting around in the open sea.”

“Yeah. They think one of our ancestors ate the ancestral mitochondria. Only, for some reason it didn’t digest it that time. It let the thing stay and work for it, instead.

Carl looked up at Saul, seriously. “This is what you mean by symbiosis, isn’t it’? The early mitochondria provided more efficient energy conversion for the host cell. And in return, it never had to hunt for food again. The host cell.”

“—Our ancestor.”

“—took care of that from then on.”

“And when one divided, so did the other, passing the arrangement down to each daughter cell. The partnership was inherited, generation by generation.” Saul nodded. “The same seems true of chloroplasts, the organelles in plant cells that do the actual work of photosynthesis. They’re kin to blue-green algae. And many other cellular components show signs they may have once been independent creatures, too.”

“Yes. I do remember reading about that.” Carl seemed interested for the first time. Saul remembered some of the conversations they’d had back in the early days, before their differences had yawned like gulf between them. He wondered if Carl missed them s much as he did.

Probably more. After all, I have Virginia.

“The same holds for the entire organism, Carl. A normal human being has countless species of creatures living in him, depending on him as he depends on them. From gut bacteria that help us digest our food, to a special type of mite that lives only at the base of human eyelashes, scouring them, eating decayed matter and keeping them clean.”

Saul spread his hands. “None of these symbiotic animals can live independently of man anymore. Nor can we very easily do without them. They’re almost as much parts of the colony organism called Homo sapiens as human DNA itself.”

Carl blinked, as if trying to absorb this new leap. “It’s like a quantum field in physics, then. The boundaries of what I call ‘me’ are… are…”

“Are amorphous. Nebulous. Difficult to define. You’ve got it! They’ve found that married couples share much the same suite of intestinal flora, for instance. Make love to a woman, and you exchange symbionts. In a sense, you become partly the same creature by sharing elements that grow and participate in each other.”

Carl frowned. And Saul realized that he was skirting a touchy subject. He hurried on.

“But here is my main point. Carl. Probably few, if any, of these symbionts simply settled into their niches without an initial struggle. Evolution doesn’t work that way… at least not usually.”

“But—”

“Every symbiont, from digestion helper to follicle cleaner, started out as an invader, once upon a time. Every synergism began in a disease.”

“I don’t…” Carl frowned in concentration. “Wait. Wait a minute.” His brow was knitted with tight furrows. “You spoke of disease as negotiation between a host and an invading.”

“— Visiting—”

“— species. But… but even if that’s the case, this negotiation takes place over the bodies of uncounted dead of both sides!” Carl looked up, eyes flashing. “True, they may come to a modus vivendi someday, but that doesn’t help the individuals who die, often horribly, broken on the wheel of evolution.”

Saul stared, unable to hide his surprise. In his most pensive moments, Carl Osborn seemed to have come upon a new facility with words. With tempering, an awkward youth had turned into something of a poet.

“Well said.” Saul nodded. “And that’s exactly what we’re seeing here on Halley. Some die abruptly. Others fight the interlopers to a standstill. Some even profit a little from some side effect of their infestation.”

Carl slapped the desktop with a loud report and swiveled to face Saul fully.