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.

A genetic analysis of the bird flu in the body of a Louisiana man who contracted the first severe case found it mutated inside of his body, a development that could allow the virus to more easily spread to humans.

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.