Dairy cattle in Nevada have been infected with a new type of bird flu that's different from the version that has spread in herds since last year. U.S.

The finding indicates that the virus, known as H5N1, has spilled from birds into cows at least twice — leading to these two sets of infections — and that it could continue to do so. It also suggests that the virus may pose a persistent risk to cows and to the people who work closely with them.