All four flu pandemics — in 1918, 1957 and 1968 and the swine flu pandemic of 2009 — were caused by Type A viruses. Influenza B viruses, however, only circulate in humans.

The mutations in the bird flu, or H5N1, viruses that caused severe infections in British Columbia and Louisiana both occurred in a protein on the surface of the virus that allows it to attach to ...

"When a flu virus infects a cell, its genetic material goes in as multiple different segments or pieces," she explained in the video, posted November 18, 2024.

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.