Marc Johnson saw trouble in the water.

Dr. Johnson, a virologist at the University of Missouri, had spent much of 2020 studying sewage, collecting wastewater from all over the state and analyzing it for fragments of the coronavirus. People with Covid-19 shed the virus in their stool, and as the coronavirus spread throughout Missouri, more and more of it began to appear in the state’s wastewater.

In January, Dr. Johnson spotted something new in his water samples: traces of B.1.1.7, a more contagious variant that was first detected in Britain. Officially, the state had no confirmed cases of B.1.1.7, but the wastewater suggested that the variant had arrived. By the end of the month, the B.1.1.7 levels in Dr. Johnson’s water samples had risen sharply, and in early February, the state finally found its first case. It has since found hundreds more.