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.

Here’s what to know about the United States’ first confirmed severe case of H5N1 avian influenza—and how to protect yourself.

A genetic analysis suggests the bird flu virus mutated inside a Louisiana patient who contracted the first severe case of the illness in the United States. The U.S.