Rising H5N1 bird flu cases in the U.S. prompted the CDC to study the virus in ferrets, revealing it may have the potential to spread and cause severe symptoms in other mammals.

New CDC research suggests that a small but significant percentage of dairy farmers working near infected cows have recently contracted H5N1, often without knowing.

The data shows that those who are infected with H5N1 may not show symptoms, Nirav Sha, the CDC’s principal deputy director, said during a call with reporters. “That means that we in public health need to cast a wider net in terms of who is offered a ...