How H5N1 bird flu hid unrecognized for weeks in dairy cattle (opens in new tab)

When H5N1 bird flu first began infecting U.S. cattle in early 2024, diagnosis was elusive because, in cows, the disease looked completely different. Instead of affecting the lungs, as H5N1 does in other mammalian species, it caused severe infection in the cows' udders, largely sparing the lungs.