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 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.

Federal health officials are recommending expanded testing of dairy farm workers after a new study found evidence of previously undetected cases of bird flu in humans. Why it matters: The study is leading officials to recommend the testing of all workers ...