Scientists believe the mutations may allow the virus to better bind to receptors in the upper airways of humans.

Tiny genetic alterations could help the bird flu virus enter cells in the upper respiratory tract, the C.D.C. said. But there is no sign that mutations are widespread in nature.

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.