shitting in the water supply
''Honey, where ya goin?"
"Gonna take a dump in the well, dear, brb"
Ironically this is exactly how the cause of cholera was discovered. Some savvy bloke mapped the locations of all the cholera victims in London, noted most of them were in a few tight clusters, walked to the geographic center of all those clusters, and behold there was a communal well that was notably shallower than normal at all of them. Shallow enough that shit-water could leach down through the ground to it.
And this is why we have modern sanitation and arent dying from cholera anymore thanks to England.
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