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.

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

A genetic analysis of samples from the patient in Louisiana recently hospitalized with the country’s first severe case of H5N1 bird flu show the virus likely mutated in the patient to potentially become more transmissible to humans,