IN LATE 2021, TUFTS UNIVERSITY virologist Wendy Puryear began to worry. The avian flu virus was behaving oddly. It usually passes through wild birds with little harm. But in Europe, a highly pathogenic strain known as H5N1 was killing a range of avian species and infecting mammals like otters and foxes.
Then it hopped across the Atlantic. Carried long distances by migrating waterfowl, gulls, and shorebirds, within months the virus found almost every corner of North America—and
