When, where and how the H5N1 bird flu virus may evolve and its capacity to spark a pandemic is hard to predict — in part, some researchers say, because of federal restrictions on gain-of-function research.

The United States has confirmed its first known severe human infection of H5N1 bird flu, in a person in Louisiana believed to have contracted the virus through contact with sick or dead birds in a backyard flock.

The Centers for Disease Control and Prevention confirmed Wednesday that the first case of severe human illness for H5N1 bird flu occurred this week in Louisiana, marking the first hospitalization for the disease in the United States.