every 48 years."

"A fever that lasted 48 years would be quite a fever," Dr. Forth said
with the shadow of a grim smile. "Nevertheless that's the only name we
have so far. Name it and you can have it. Allison's disease?"

Jay Allison greeted this pleasantry with a repressive frown. "As I
understand it, the disease cycle seems to be connected somehow with the
once-every-48-years conjunction of the four moons, which explains why
the Darkovans are so superstitious about it. The moons have remarkably
eccentric orbits--I don't know anything about that part, I'm quoting Dr.
Moore. If there's an animal vector to the disease, we've never
discovered it. The pattern runs like this; a few cases in the mountain
districts, the next month a hundred-odd cases all over this part of the
planet. Then it skips exactly three months without increase. The next
upswing puts the number of reported cases in the thousands, and three
months after _that_, it reaches real pandemic proportions and decimates
the entire human population of Darkover."

"That's about it," Forth admitted. They bent together over the folder,
Jay Allison drawing back slightly to avoid touching the other man.

Forth said, "We Terrans have had a Trade compact on Darkover for a
hundred and fifty-two years. The first outbreak of this 48-year fever
killed all but a dozen men out of three hundred. The Darkovans were
worse off than we were. The last outbreak wasn't quite so bad, but it
was bad enough, I've heard. It has an 87 per cent mortality--for humans,
that is. I understand the trailmen don't die of it."

"The Darkovans call it the trailmen's fever, Dr. Forth, because the
trailmen are virtually immune to it. It remains in their midst as a mild
ailment taken by children. When it breaks out into the virulent form
every 48 years, most of the trailmen are already immune. I took the
disease myself as a child--maybe you heard?"

Forth nodded. "You may be the only Terran ever to contract the disease
and survive."

"The trailmen incubate the disease," Jay Allison said. "I should think
the logical thing would be to drop a couple of hydrogen bombs on the
trail cities--and wipe it out for good and all."

(Sitting on the Sofa in Forth's dark office, I stiffened with such fury
that he shook my shoulder and muttered, "Easy, there, man!")

* * * * *

Dr. Forth, on the screen, looked annoyed, and Jay Allison said, with a
grimace of distaste, "I did