It is not clear whether the sewage systems that have not reported such dumping are doing any better, because data on overflows and spillage are often incomplete.

As cities have grown rapidly across the nation, many have neglected infrastructure projects and paved over green spaces that once absorbed rainwater. That has contributed to sewage backups into more than 400,000 basements and spills into thousands of streets, according to data collected by state and federal officials. Sometimes, waste has overflowed just upstream from drinking water intake points or near public beaches.

There is no national record-keeping of how many illnesses are caused by sewage spills. But academic research suggests that as many as 20 million people each year become ill from drinking water containing bacteria and other pathogens that are often spread by untreated waste.

A 2007 study published in the journal Pediatrics, focusing on one Milwaukee hospital, indicated that the number of children suffering from serious diarrhea rose whenever local sewers overflowed. Another study, published in 2008 in the Archives of Environmental and Occupational Health, estimated that as many as four million people become sick each year in California from swimming in waters containing the kind of pollution often linked to untreated sewage.

Around New York City, samples collected at dozens of beaches or piers have detected the types of bacteria and other pollutants tied to sewage overflows. Though the city’s drinking water comes from upstate reservoirs, environmentalists say untreated excrement and other waste in the city’s waterways pose serious health risks.

A Deluge of Sewage

“After the storm, the sewage flowed down the street faster than we could move out of the way and filled my house with over a foot of muck,” said Laura Serrano, whose Bay Shore, N.Y., home was damaged in 2005 by a sewer overflow.

Image William Grandner, superintendent of Owls Head Water Pollution Control Plant in Brooklyn, kept an eye on multiple monitors that track the flow of sewage. Credit... Damon Winter/The New York Times

Ms. Serrano, who says she contracted viral meningitis because of exposure to the sewage, has filed suit against Suffolk County, which operates the sewer system. The county’s lawyer disputes responsibility for the damage and injuries.