When the virologist Kirsten St. George learned last summer that a paralyzed patient in New York’s Rockland County had tested positive for polio, she turned her attention to the state’s sewers. Polio is particularly stealthy because most infected people never develop symptoms but can still spread the virus. A wastewater-surveillance network established during the Covid-19 pandemic helped officials at the New York State Department of Health’s Wadsworth Center track polio’s spread in several counties.

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