A genetic analysis suggests the bird flu virus mutated inside a Louisiana patient who contracted the nation’s first severe case of the illness, the U.S. Centers for Disease Control and Prevention said this week.

Growing concerns surrounding the transmission of H5N1 bird flu. The CDC reports that samples collected from a severely ill patient in Louisiana showed signs of mutation, as they differ from the samples collected from sick and dead birds believed to have infected the man in the first place.

The Centers for Disease Control and Prevention (CDC) has identified mutations in the H5N1 bird flu virus that may increase its ability to infect humans. These mutations, found in the hemagglutinin (HA) gene,