Anne Sofie Hammer was searching for sick mink. The Danish government had hired the University of Copenhagen veterinary pathologist in June 2020 to investigate if farmers were infecting mink with the novel coronavirus. This meant going from farm to farm, looking for animals that weren’t eating or had a cough and taking blood samples and mouth swabs.

The virus, SARS-CoV-2, had been spreading rapidly among mink farmers, and officials were worried that it would not only infect the country’s roughly 17 million farmed mink but also jump back to humans. That leap had already been documented in the Netherlands.

Gasping for breath, chests heaving, some older mink especially were struggling to survive, she found. As their immune systems