So much was normal reasoning. Now the strictly technical logic of the Med Service took over to explain the current state of things. In one century or less an isolated community could lose, absolutely, its defenses against diseases to which it was never exposed. Amerinds were without defense against smallpox, back on Earth. A brown race scattered among thousands of tiny islands was nearly wiped out by measles when it was introduced. Any contact between a long-isolated community and another--perhaps itself long-isolated--would bring out violently any kind of contagion that might exist in either. There was the mechanism of carriers. The real frequency of disease-carriers in the human race had been established less than two generations ago. A very small, isolated population could easily contain a carrier or carriers of some infection. They could spread it so freely that every member of their group acquired immunity during infancy. But a different isolated group might contain a carrier of a different infection and be immune but distributive of it. It was literally true that each of the three cities might have developed in their first century of isolation a separate immunity to one disease and a separate defenselessness against all others. A member of one community might be actually deadly to a member of either of the others whom he met face to face. With icy wind blowing upon him as the sledge rushed on, Calhoun wryly realized that all this was wholly familiar. It was taught, nowadays, that something of the sort had caused the ancient, primitive human belief that women were perilous to men, and a man must exercise great precaution to avoid evil _mana_ emanating from his prospective bride. When wives were acquired by capture and all human communities were small and fiercely self-isolated--why each unsanitary tribal group might easily acquire a condition like that assumed in cities One, Two, and Three. The primitive suspicion of woman would have its basis in reality if the women of one tribe possessed immunity to some deadly microbe their skin or garments harbored--and if their successful abductors had no defense against it. The speeding sledge swerved. It leaned inward against the turn. It swerved again, throwing monstrous sheets of snow aloft. Then the drive-jet lessened its roar. The shimmering bow-waves ceased. The sledge slowed to a mere headlong glide. "Their trail!" Hunt cried in Calhoun's ear. Calhoun saw depressions in the snow. There were two sets of pear-shaped dents in the otherwise virgin surface. Two man beings, wearing oblong frames on their feet, criss-crossed with cordage to support them atop the snow, had trudged ahead, here, through the gold-blue night. * * * * * Calhoun knew exactly what had happened. He could make the modifications the local situation imposed upon a standard pattern, and reconstitute a complete experience leading up to now.