"There were papers in your clothes—what was left of them. And—it'll be hard to explain all this. I've only been here a month myself."
Vanning rubbed his stubbly beard. "We're on Venus?"
"Yes, of course. This is a—a valley. The Swamja have lived here for ages, since before Earthmen colonized Venus."
"I never heard of them."
"None ever return from this place," Lysla said sombrely. "They become slaves of the Swamja—and in the end they die. New slaves come, as you did."
Vanning's eyes narrowed. "Hold on. I'm beginning to understand, a little. The Swamja—those fish-headed people—have a secret city here, eh? They're intelligent?"
She nodded. "They have great powers. They consider themselves the gods of Venus. You see—Jerry Vanning—they evolved long before the anthropoid stock did. Originally they were aquatic. I don't know much about that. Legends ... Anyway, a very long time ago, they built this city and have never left it since. But they need slaves. So they send out the North-Fever—"
"What?" Vanning's face grayed. "Lysla—what did you say? The fever's artificial?"
"Yes. The virus is carried by microscopic spores. The Swamja send it out to the upper atmosphere, and the great winds carry it all over Venus. The virus strikes very quickly. Once a man catches it, as you did, he goes north. These mountains are a trap. They're shaped like a funnel, so anyone with the fever inevitably heads into the pass, as you did. They are drawn through the mirage, which looks like a wall of rock. No one who wasn't—sick—would try to go through that cliff."