Bird flu, or avian influenza, is an infectious disease of birds caused by type A strains of the influenza virus. Bird flu epidemics have occurred worldwide. The recent spread of bird flu has been localised to certain parts of Asia.
Migratory waterfowl - most notably wild ducks - are the natural reservoir of bird flu viruses. It is suspected that infection can spread from wild fowl to domestic poultry, including chickens and turkeys, and then to humans. Live bird markets have also played an important role in the spread of epidemics.
The type currently causing concern is the `highly pathogenic` Asian strain of the H5N1 virus. Scientists have discovered four different subtypes of H5N1, and there could well be more. However, all are deadly to birds, and can cause disease - and death - in humans. However, it is important to note that H5N1 is overwhelmingly a disease that affects birds - and not humans.
It is true that humans have been infected, but almost all have been poultry workers who have come into intimate contact with birds. H5N1 cannot pass easily from human to human.