The bird flu has been infecting American flocks for years, leading to a series of infections and increased egg prices. But now, there's a new twist: A different strain has been detected in U.S. poultry.

H5N9 is a rare subtype of the influenza A virus that can cause highly pathogenic avian influenza (HPAI), also known as bird flu. It's a reassortment strain that originated from the H5N1, H7N9, and H9N2 subtypes.