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

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. Nearly all were in contact with infected dairy cows or infected poultry.