California officials have declared a state of emergency over the spread of bird flu, which is tearing through dairy cows in that state and causing sporadic illnesses in people in the U.S.

Gov. Gavin Newsom’s declaration has riled up concern, particularly in other top dairy-producing states, such as Wisconsin and Pennsylvania.

Bird flu virus has swept through farms across the nation’s largest dairy producing state. All human cases had been mild - until now.