An examination reveals the aliens weren’t defeated by a toxin, and they weren’t defeated by Earth’s atmosphere, germs, water, or any of the usual culprits. They were defeated by time. Specifically, time runs faster for them. They simply fell to old age before the battle was done.
There’s a throwaway reference to “tachyons,” science fiction’s favorite temporal explanation, but it’s otherwise accepted at face value that time runs at a different rate wherever they come from, and the effect of that followed them.
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