All he had to do now was materialize in the right spot, flash out and pull the red switch toward him. Then his well-paid assignment would be done.
But....
He stopped and scratched his dirt-matted hair. Wasn't there something he was supposed to do a second before materialization? Yes, that useless old windbag, Sadha, had given him a last instruction.
He picked up the sealed metal cylinder, walked to the entrance of the time machine and tossed it into the gray murk. A solid object floating near the entrance caught his eye. He put his arm out—whew, it was cold!—and pulled it inside.
A small metal box. Funny. What was it doing out there? Curiously, he opened it, hoping to find something valuable. Nothing but a few sheets of paper, Alben noted disappointedly. He began to read them slowly, very slowly, for the manuscript was full of a lot of long and complicated words, like a letter from one bookworm scientist to another.
The problems all began with the Guided Missile Experiment of 1976, he read. There had been a number of such experiments, but it was the one of 1976 that finally did the damage the biologists had been warning about. The missile with its deadly warhead exploded in the Pacific Ocean as planned, the physicists and the military men went home to study their notes, and the world shivered once more over the approaching war and tried to forget about it.
But there was fallout, a radioactive rain several hundred miles to the north, and a small fishing fleet got thoroughly soaked by it. Fortunately, the radioactivity in the rain was sufficiently low to do little obvious physical damage: All it did was cause a mutation in the mumps virus that several of the men in the fleet were incubating at the time, having caught it from the children of the fishing town, among whom a minor epidemic was raging.
The fleet returned to its home town, which promptly came down with the new kind of mumps. Dr. Llewellyn Shapiro, the only physician in town, was the first man to note that, while the symptoms of this disease were substantially milder than those of its unmutated parent, practically no one was immune to it and its effects on human reproductivity were truly terrible. Most people were completely sterilized by it. The rest were rendered much less capable of fathering or bearing offspring.
Shapiro's Mumps spread over the entire planet in the next few decades. It leaped across every quarantine erected; for a long time, it successfully defied all the vaccines and serums attempted against it. Then, when a vaccine was finally perfected, humanity discovered to its dismay that its generative powers had been permanently and fundamentally impaired.