Raymond Z Gallun's "Seeds of the Dusk" (novelette, biological warfare): How Martians exterminated the descendants of humanity
While I could pick many holes in the story, they're irrelevant. I'll any day put this story among the very best science fiction has on offer.
Story summary.A single seed of sentient Martian plant has reached earth - of the millions thrown from there. But that is enough.
Story is set billions of years in future, when earth is near the dusk of its habitability. But that's the time Martians find the world in its prime.
Ensuing conflict of alien with human-descended & barbaric Itorloo will end with extermination of Itorloo. Aliens' technique: trap a single Itarloo specimen, study his body chemistry, manufacture a fatal-only-to-Itorloo highly contagious germ with a long gestation period, & release the specimen back in the wilds to infect everyone else.
- Raymond J Healy & J Francis McComas (Eds)' "Adventures in Time and Space".
Fact sheet.First published: Astounding, June 1938.
Listed among the stories from John Campbell's Astounding/Analog.