The bird flu Avian (bird) flu is caused by influenza avian viruses that occur naturally among birds.  There are many different subtypes of type influenza viruses.  All known subtypes can be found in birds.  The avian flu currently of concern is the H5N1 subtype.  Avian H5N1 flu in humans is currently very limited and not a pandemic.

With the flu spreading around the world, the virus has turned up in birds in Asia and Europe, and, most recently, Africa. So far, bird flu has only been passed from birds either to other birds or, in isolated cases, to humans. However, if the flu mutates into a strain that can pass from human to human, people will have no immunity and the flu will probably pass rapidly from person to person, creating a pandemic. Flu vaccines can only be made to protect against a particular virus, and, since the virus has yet to be passed from human to human, no vaccine can be developed.