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. After someone in southwest Louisiana was hospitalized with a severe case of bird flu, the first such illness reported in the United States, health workers swabbed the person’s nose and throat, looking for genetic clues about the virus.
On Thursday, federal health officials reported some unsettling results. Some of the genetic samples contained mutations that in theory might help the bird flu virus, H5N1, infect people more easily.
One of those mutations was reported last month in a viral sample taken from a teenager with a severe case of bird flu in British Columbia, Canada. The teenager was placed on a ventilator during a long hospitalization.