He looked at her carefully. With increasing worry, he saw that her nose was red. Her long eyelashes were blinking back telltale moisture. And yet it didn't seem as though she'd been crying or anything. Were these, then, more forerunners of the plague? Several other men sneezed violently. And Ron looked, with a touch of real fear, at the motionless body of a Callistan, lying on the grass nearby, its fur blowing in the wind. Maybe the Acharian doom was also going to be an Earthian doom. "Anna--" Ron gasped. That single name, as he uttered it now, was like some strange plea and prayer to the unknown. But the girl smiled back at him. "I think that I'm the one who understands what this is all about this time, instead of you, Ron," she declared almost tauntingly. "Then tell us, Miss!" Bart Mallory urged in a half-frantic tone. Anna glanced briefly and mysteriously at the bulk of Saturn--a pale, pearly, enhaloed bubble at the horizon--above the now green-tinted hills. "Yes, it's Arne's scheme come true," she said musingly. "The Acharians lived unknowingly with death here, for almost two terrestrial months. Too few of them had ever visited Earth, even to recognize the enemies that lurked there for them. And when that nemesis was brought here, it was far too harmless and unobtrusive in its aspect, for them to notice or be warned. "Remember what Arne Reynaud told us, long ago, just before he was killed by one of those Callistan heat-guns, in front of the Leiccsendale Community Bank? The time he made his speech, Ron? You heard him, too, Mr. Mallory. I think I can quote almost his exact words: "'Me and a brother of mine are probably the only men, Earthian or Callistan, who realize why Callistans get very sick on Earth at certain times--though it's simple.... I saw one die once, there, in summer. It ain't just the density of the air. They can stand that. Something else. I found out...." "Well, what is it, then?" Mallory demanded, not meaning to sound impatient. * * * * * The girl glanced at him, then back at Ron, then all around at the waiting faces. "We all know, don't we," she said, "that we are used to certain conditions, we Terrestrials from Earth. We get tough and acclimated. People from other worlds, not used to similar conditions, wouldn't have the same resistance. Space travel bears this out--Martian plagues spreading on Earth--Venusians dying of the common cold. Even an interchange of germs between the terrestrial continents was dangerous, according to history. Tuberculosis ravaging the American Indians. Eskimoes killed by the measles. Terrestrial germ diseases don't bother the Callistans, it is true, because their blood is at too high a temperature for Earthly bacteria to survive. But there's another thing--a weak point. The cargo Ron and I brought from Mars in the _Barbarian_, was the answer."