“A viroid.”

“What’s that?”

“A bare RNA fragment. Smallest known infectious agent. Strange.”

“Ka.” Nirgal felt his stomach clamping inward. “How did it get here?”

“On a parasite, probably. This kind seems to be infecting grass. We need to find out.”

So they gathered samples, and drove back down to Sabishii.

Nirgal sat on a futon on the floor of Tariki’s living room, feeling sick. Tariki and Sax talked long after dinner, discussing the situation. Other viroids had been appearing in a rapid dispersal from Tharsis; apparently they had made it across the cordon sanitaire of space, arriving on a world that had been previously innocent of them. They were smaller than viruses, much smaller, and quite a bit simpler. Nothing but strands of RNA, Tariki said, about fifty nanometers long. Individuals had a molecular weight of about 130,000, while the smallest known viruses had molecular weights of over a million. They were so small that they had to be centrifuged at over 100,000 g in order to be pulled out of suspension.

The potato-spindle-tuber viroid was well understood, Tariki told them, tapping around on his screen and pointing at the schematics called up. A chain of merely 359 nucleotides, lined out in a closed single strand with short double-strand regions braiding it. Viroids like this one caused several plant diseases, including pale cucumber disease, chrysanthemum stunt, chlorotic mottle, cadang-cadang, citrus exocortis. Viroids had also been confirmed as the agent in some animal brain diseases, like scrapie, and kuru, and Creutzfeldt-Jakob disease in humans. The viroids used host enzymes to reproduce, and then were taken to be regulatory molecules in the nuclei of infected cells, disturbing growth-hormone production in particular.

The particular viroid in Nirgal’s basin, Tariki said, had mutated from potato spindle tuber. They were still identifying it in the labs at the university, but the sick grass made him sure they were going to find something different, something new.

Nirgal felt sick. The names of the diseases alone were enough to do it. He stared at his hands, which had been plunged thick in infected plants. Through the skin, into the brain, some kind of spongiform encephalopathy, mushroom growths of brain blooming everywhere.

“Is there anything we can do to fight it?” he said.

Sax and Tariki looked at him.

“First,” Sax said, “we have to find out what it is.”

• • •

That turned out to be no simple matter. After a few days, Nirgal returned to his basin. There he could at least do something; Sax had suggested removing all the potatoes from the potato fields. This was a long dirty task, a kind of negative treasure hunt, as he turned up diseased tuber after tuber. Presumably the soil itself would still hold the viroid. It was possible he would have to abandon the field, or even the basin. At best, plant something else. No one yet understood how viroids reproduced; and the word from Sabishii was that this might not even be a viroid as previously understood.