More than 9 million birds in Minnesota's commercial and domestic poultry flocks have died during the 2015 avian influenza outbreak sweeping the western half of the United States. The birds were killed by the flu itself or euthanized to prevent the spread of the disease.

The virus is so virulent that thousands of birds can die in a few days.

Avian flu in Minnesota: Bird deaths by county

The state has verified 108 outbreaks among chicken, turkey and mixed-poultry flocks in 23 counties, many of which lie in the heart of Minnesota's turkey-producing country. (Map data via Minnesota Board of Animal Health)

The outbreak has hit Minnesota's $800 million-plus turkey industry especially hard: Nearly 5 million birds have died from the disease or were killed to prevent the virus from spreading, mostly in the southwestern quadrant of the state.

Minnesota chicken producers haven't escaped the disease untouched, either: More than 4 million chickens have died because of it. A relative handful of birds in a backyard mixed-poultry flock were also hit.

Full coverage: Minnesota and the bird flu outbreak

The Minnesota Board of Animal Health and the U.S. Department of Agriculture are tracking cases of avian flu across the state. When an outbreak is confirmed, the agencies note the size of each farm and the county in which it's located. Affected farms are not identified by name.

Throughout the outbreak, officials have emphasized that the food system is safe. Still, about four dozen countries have banned Minnesota poultry, including the entire European Union and most of Central America, according to the U.S. Department of Agriculture.

So far, no human cases have been reported in the United States or elsewhere, according to the USDA. Although the risk of transmission to humans is low, workers in the state who have contact with the infected flocks will be monitored for the illness.