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.

The H5 bird flu is widespread in wild birds and is causing outbreaks in U.S. poultry and dairy farms, the CDC says. There have been outbreaks in 443 U.S. dairy herds, and 48 states have reported outbreaks in poultry.