Influenza naturally infects wild birds all around the world, although they usually do not become ill. The virus is very contagious, however, and it can become a problem when the virus is transmitted to domesticated birds, such as chickens, ducks, or turkeys, because domesticated poultry can succumb to illness and death from influenza. Since 1997, H5N1 infections in birds have spread, initially throughout Asia. Then as birds traveled along their migratory routes, H5N1 dispersed to Russia and Europe, and later to countries in the Middle East and on the African continent.
One reason why avian H5N1 is not readily transmissible among people has to do with the hemagglutinin, or HA, protein of the virus that determines which cell type the virus can enter. human influenza viruses, which infect cells high in the respiratory tract, the H5N1 HA protein attaches to cells much lower in the respiratory track. The virus is so deep within the respiratory tract that it is not coughed up or sneezed out, and so it does not easily infect other people. If the HA protein of H5N1 were to mutate so that it could infect cells higher in the respiratory tract, then it would more likely be able to pass from person to person.
The H1N1 virus continues to circulate at low levels, but it is no longer the dominant influenza strain, and its behavior more closely resembles a seasonal influenza virus than a pandemic flu.
- Human Influenza & Avian Influenza