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 U.S. Centers for Disease Control and Prevention says a patient has been hospitalized with a severe case of H5N1 infection in Louisiana, making it the first instance of a severe illness linked to the virus in the United States.

A Louisiana patient is critically ill with severe respiratory symptoms from H5N1 bird flu, marking the 61st U.S. human case this year. California has declared a state of emergency. CBS News medical contributor Dr.