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.

The agency, in its latest update released Thursday, said the mutations were not found in samples taken from wild birds, suggesting the virus changed over the course of the patient’s infection.