Authorities believe wild birds are behind the first big outbreak of bird flu in the United States, which is slamming commercial producers of turkey and chickens in the Midwest.

The H5N2 virus has infected more than 13 million birds since early last month, and disease experts believe it came from European and Asian strains of the disease, according to an Associated Press report.

The U.S. Department of Agriculture and the U.S. Geological Survey have so far identified three versions of bird flu, and a total of 57 cases since December, which at first began to infect backyard flocks as well as wild captive and aquatic birds on the West Coast. After that, more cases began to pop up early this year, and they spread to the Midwest by March, according to the report.

Bird flu has been detected in 100 species of wild birds, but most of them don’t cause any major sickness. However, they are left behind in feces and feathers, as well as on the bodies of birds that have died, and the infection can be spread to ducks, geese, and raptors.

Authorities aren’t quite sure how the virus is spreading into commercial flocks, which are typically shut off from the outside world, but it could come in through the clothing of workers despite the fact that the farms should have biosecurity measures in place. Or, wind could be carrying infected dirt particles or feathers into ventilation openings.

This is the first widespread case of bird flu that is infecting millions of commercial birds.

There have been no cases confirmed yet of the virus causing any problems for humans, as scientists don’t think H5N2 can attach to human cells easily, but there is a chance that it could happen as the virus can mutate into forms with a higher likelihood of infection.

The Centers for Disease Control and Prevention advises keeping your distance from wild birds and to not contact surfaces that look like they have bird feces on them. Also, don’t handle domestic poultry that appears to be sick or has died, and always thoroughly cook poultry products.