A new study found some dairy workers showed signs of infection, even when they didn’t report feeling sick. Blood tests from 115 farmworkers in Colorado and Michigan found eight had antibodies that indicated previous infection with the virus.

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.

Bird flu (H5N1) has been spreading across the country, among both animals and people, since April of this year. As the Centers for Disease Control and