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.

About 7% of dairy workers exposed to the avian flu that is spreading through U.S. herds have become infected themselves, federal experts estimate. That number highlights concerns that circulating strains of bird flu might become highly infectious in humans,