Necator americanus (Hookworm) and latrines.
Long ago, people in the Southern United States would just go #2 by going out to a field and squatting. The problem is that there are parasites, namely hookworm, that can crawl out of old poo and reinfect people. While a few hookworms aren't that bad, too many can cause severe lethargy.
Enter the solution: a pit latrine. Dig a hole deep enough so the hookworms can't find another foot to invade before they die, and problem solved.
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