Since the start of the virus outbreak, the number of reported coronavirus cases has largely relied on testing. But it’s always been believed to be an imperfect system. After all, there have been asymptomatic cases, mild cases in which infected people might not seek testing or outside treatment, unequal access to health care from community to community and the increasing demand of at-home rapid tests, of which results may not be reported to health officials.

But scientists in several states, as well as some other countries, are also analyzing wastewater, which could be a more reliable gauge.

SARS-CoV-2, the virus that causes COVID-19, can be shed in an infected person’s feces. Sewage samples at wastewater treatment plants can help tell the story of how widespread the virus is in a community at a given time, and regular testing can show whether the virus has become more or less prevalent over time.