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.

The bird flu virus found in a severely ill patient hospitalized in the United States has mutated to become better adapted to human airways, though there is no evidence it has spread beyond the individual,

A new mutation and the first severe case of H5N1 bird flu discovered in a human in Louisiana has raised concern among some experts of a pandemic.