H5N1 bird flu, also known as avian influenza, continues to spread, with reports of a third human case from an unknown exposure source. To date, 67 cases have been confirmed, and one death has been reported in the U.

The California duck farm's strain of H5N9 is different from those earlier cases because this time it is closely related to a newer variant of the H5N1 virus. That variant is called H5N1 D1.1, the USDA spokesperson said, which has emerged over the past year to dominate U.S. cases in wild birds and poultry.