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.

The Centers for Disease Control and Prevention (CDC) has identified mutations in the H5N1 bird flu virus that may increase its ability to infect humans. These mutations, found in the hemagglutinin (HA) gene,

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.