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According to the introductory notes in Voyagers in Time, the original magazine
appearance of this story was mutilated by an overly zealous editor.
The story made its debut as it was originally written
in that collection. It's a nifty little time-travel paradox story. In the 28th Century, the Earth has been rid of disease, so germ-ridden time travelers from the past are quarantined at a prison on the moon where they can't infect the world's unprotected population.
Silverberg has written about the genesis of the story, including its indebtedness to Heinlein's By His Bootstraps
.
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