The CDC warns samples of the bird flu virus show signs of mutations that could make it more transmissible to humans. The CDC says it appears the H5N1 virus contained mutations in the Louisiana patient who contracted the nation’s first severe case of the illness.

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.

A genetic analysis suggests the bird flu virus mutated inside a Louisiana patient who contracted the nation’s first severe case of the illness, the U.S. Centers for Disease Control and Prevention said this week.