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.

The CDC said that testing of a person with a severe case of bird flu indicated the virus had mutated but the risk remains low to the public as no human-to-human transmission has been documented.

The patient was likely infected after having contact with sick and dead birds in a backyard flock, the US Centers for Disease Control and Prevention said earlier this month.