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.

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.

Public health officials are sounding the alarm after bird flu mutations were discovered in a Louisianan patient. Dozens of milder cases have been recorded in the U.S. this year.