British public-health officials were mystified. Nationwide restrictions were driving down the spread of coronavirus across much of the country by late November. But in one part of South-east England infections were inexplicably surging.

Epidemiologists set out to investigate, at first assuming there had been some kind of superspreader event or that people were ignoring social-distancing rules at work, in illegal house parties, or elsewhere. They found nothing. Stumped, they turned to a team of scientists monitoring mutations in the virus’ genome.