Bird Flu – Not a Laughing Matter Bird flu is a highly infectious disease caused by the H5N1 virus. The current strain of avian influenza has wiped-out millions of birds but has difficulty crossing the species barrier into humans. In the human cases that have occurred in Asia and Europe, more than half of the people died. There is fear that the virus might mutate and begin to be easily transmitted from person to person causing an influenza pandemic like the one of 1918.