The world is eager to come out of lockdown. But if countries simply return to business as usual, new outbreaks of Covid-19 will follow. The only solution that public health experts see is to keep careful track of the coronavirus and clamp down on new flare-ups.

The trouble is that the most obvious way to monitor the virus — testing person by person — has already proved to be a huge, expensive challenge. Experts say we’re nowhere near the scale we need to get a good picture of the pandemic.

Now some scientists are looking for the virus not in our noses, blood or spit, but somewhere else: in our sewers.

“It’s the signature of a whole community,” said Krista Wigginton, an environmental engineer at the University of Michigan who has been finding the coronavirus in wastewater around the Bay Area in California.