For Halden, who for decades had been a lonely evangelist for the emerging field of wastewater-based epidemiology, the story of Guadalupe illustrates the potential he believes it has to transform public health in the United States — by creating a near-real-time monitoring network that could identify not just Covid-19 and drug use within a community, but also other pathogens and even chronic conditions like cancer, Alzheimer’s, and mental health disorders. “We’ve seen from our data how important it is to have a fine enough resolution that you can catch clusters of infections in specific areas, like Guadalupe,” said Halden. “We could only have observed that by having this neighborhood-by-neighborhood network.” In the last year, the nation’s enthusiasm has finally caught up to his own. As the pandemic spread, hundreds of U.S. cities, states, prisons, universities, and private businesses leaped, sometimes clumsily, into wastewater surveillance. Federal investments in validating the science and building out a standardized national system followed. With vaccines driving SARS-CoV-2 underground, the question now is, what will governments, schools, and businesses do with all that surveillance infrastructure? Tempe, operating at an unprecedented level of granularity, represents the leading edge of what’s possible. But while the technology becomes more powerful for answering scientific questions and crafting public health interventions the more finely you slice and dice the sewershed, it also raises more privacy concerns. The American wastewater surveillance revolution might have started with SARS-CoV-2, but there’s no telling yet where it ultimately will lead.