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.

The CDC's updated recommendations are based, in part, on the study of 115 dairy workers in Colorado and Michigan. Blood tests found eight workers had been infected with bird flu, but only four of those workers recalled having symptoms. The most common ...

The CDC study provides the largest window to date into how the bird virus first detected in March in dairy cows may be spreading to people. It suggests that the virus has infected more humans than the 46 farmworkers identified in the U.S. as of Thursday.