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.

Samples from a hospitalized patient in Louisiana show changes that could make the H5N1 virus spread more easily between humans

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