Janet Hostetter,File, Associated Press
In this Nov. 2, 2005 file photo, turkeys are pictured at a turkey farm near Sauk Centre , Minn. A dangerous strain has affected poultry flocks in Minnesota, Missouri, Kansas and Arkansas. The disease is carried by wild waterfowl, and authorities are trying to determine how the commercial flocks became infected.

MINNEAPOLIS — A dangerous new bird flu strain has affected poultry flocks in Minnesota, Missouri, Kansas and Arkansas. That's sending experts and farmers scrambling to determine how it arrived — and to stop it from spreading.

Avian influenza is common in wild waterfowl and doesn't normally hurt them. But this strain, H5N2, is deadly when it spreads to commercial poultry.

Most U.S. turkeys and chickens spend their entire lives indoors to protect them from diseases. But the infections show that those flocks aren't completely secure, as viruses can be tracked into barns or spread from waterfowl to shore birds that sneak into a barn.

The outbreaks have led dozens of countries to ban poultry imports from affected states. Producers are tightening biosecurity. But officials say the danger to humans is very low.