Three days before Thanksgiving, a planeload of passengers from South Africa touched down in San Francisco. One of them was a woman who was in the early stages of a Covid-19 infection, though she wouldn't know it for almost another week.

It happened to be the same day that scientists 10,000 miles away in Botswana and South Africa began alerting the world to a new and highly mutated version of the virus that causes Covid-19.

Alexandria Boehm, a professor of civil engineering at Stanford University, read about the usual pattern of mutations in the yet-to-be-named variant and sprang into action.

For more than a year, Boehm and her team of 45 people at the Sewer Coronavirus Alert Network, or SCAN, have been collecting and testing daily sludge samples from wastewater processing plants across Northern California, hunting for fragments of the new coronavirus.