NAIROBI—Throngs of traders haggle and jostle for goods along busy streets, constantly interrupted by the hooting of matatus, local public transport vehicles, and the shouting of pushcart drivers, known as mkokotenis. This neighborhood is no place for the squeamish. The matatus and the mkokotenis make their way through deep, water-filled potholes, splashing thick, dark liquid onto crowded sidewalks. Like the badly damaged roads, the sewage system in Nairobi’s Eastleigh district was built by British colonists in the 1920 s to service a few hundred working-class Africans and Indians, but now it must bear the waste of over 100,000 residents. Today raw sewage oozes out of thousands of household pipes that have ruptured after decades of neglect. The dark green sludge mixes with runoff in the streets to form a foul porridge of human excrement.

But these pools of unsanitary...

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