The CDC has confirmed a total of 66 human cases of bird flu across the United States as of Friday, although the number is believed to be higher.

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.

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 become potentially more transmissible to humans,