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.

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 ...