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"It's the smell!" -- Smith I think the easiest answer is that there is something in Earth's biology that disagrees with them... something that can be "managed" with continuous medical interventio...
Answer
#1: Attribution notice removed
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System
·
2020-12-01T17:59:13Z (almost 4 years ago)
Source: https://worldbuilding.stackexchange.com/a/161760 License name: CC BY-SA 4.0 License URL: https://creativecommons.org/licenses/by-sa/4.0/