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.

Tiny genetic alterations could help the bird flu virus enter cells in the upper respiratory tract, the C.D.C. said. But there is no sign that mutations are widespread in nature.

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.