Avian influenza, commonly known as bird flu, refers to "influenza caused by viruses adapted to birds."
"Bird flu" is a phrase similar to "Swine flu", "Dog flu", "Horse flu", or "Human flu" in that it refers to an illness caused by any of many different strains of influenza viruses that have adapted to a specific host. All known viruses that cause influenza in birds belong to the species: Influenza A virus. All subtypes (but not all strains of all subtypes) of Influenza A virus are adapted to birds, which is why for many purposes avian flu virus is the Influenza A virus (note that the "A" does not stand for "avian").
Adaption is non-exclusive. Being adapted towards a particular species does not preclude adaptions, or partial adaptions, towards infecting different species. In this way strains of influenza viruses are adapted to multiple species, though may be preferential towards a particular host. For example, viruses responsible for influenza pandemics are adapted to both humans and birds. Recent flu research into the genes of the Spanish Flu virus shows it to have genes adapted to both birds and humans; with more of its genes from birds than less deadly later pandemic strains.
Genetics
Genetic factors in distinguishing between "human flu viruses" and "avian flu viruses" include:
PB2: (RNA polymerase): Amino acid (or residue) position 627 in the PB2 protein encoded by the PB2 RNA gene. Until H5N1, all known avian influenza viruses had a Glu at position 627, while all human influenza viruses had a lysine.
HA: (hemagglutinin): Avian influenza HA bind alpha 2-3 sialic acid receptors while human influenza HA bind alpha 2-6 sialic acid receptors. Swine influenza viruses have the ability to bind both types of sialic acid receptors.
Influenza Pandemic
Pandemic flu viruses have some avian flu virus genes and usually some human flu virus genes. Both the H2N2 and H3N2 pandemic strains contained genes from avian influenza viruses. The new subtypes arose in pigs coinfected with avian and human viruses and were soon transferred to humans. Swine were considered the original "intermediate host" for influenza, because they supported reassortment of divergent subtypes. However, other hosts appear capable of similar coinfection (e.g., many poultry species), and direct transmission of avian viruses to humans is possible. The Spanish flu virus strain may have been transmitted directly from birds to humans.
In spite of their pandemic connection, avian influenza viruses are noninfectious for most species. When they are infectious they are usually asymptomatic, so the carrier does not have any disease from it. Thus while infected with an avian flu virus, the animal doesn't have a "flu". Typically, when illness (called "flu") from an avian flu virus does occur, it is the result of an avian flu virus strain adapted to one species spreading to another species (usually from one bird species to another bird species). So far as is known, the most common result of this is an illness so minor as to be not worth noticing (and thus little studied). But with the domestication of chickens and turkeys, humans have created species subtypes (domesticated poultry) that can catch an avian flu virus adapted to waterfowl and have it rapidly mutate into a form that kills in days over 90% of an entire flock and spread to other flocks and kill 90% of them and can only be stopped by killing every domestic bird in the area. Until H5N1 infected humans in the 1990s, this was the only reason avian flu was considered important. Since then, avian flu viruses have been intensively studied; resulting in changes in what is believed about flu pandemics, changes in poultry farming, changes in flu vaccination research, and changes in flu pandemic planning.
H5N1 has evolved into a flu virus strain that infects more species than any previously known flu virus strain, is deadlier than any previously known flu virus strain, and continues to evolve becoming both more widespread and more deadly causing a leading expert on avian flu to publish an article titled "The world is teetering on the edge of a pandemic that could kill a large fraction of the human population" in American Scientist. He called for adequate resources to fight what he sees as a major world threat to possibly billions of lives. Since the article was written, the world community has spent billions of dollars fighting this threat with limited success.
H5N1
As of 2007, "avian flu" is being commonly used to refer to infection from a particular subtype of Influenza A virus, H5N1, which can cause severe illness in humans who are infected. This strain is transmitted through contact with infected birds, but has only been transmitted from one person to another in a few cases. H5N1 flu is therefore not at pandemic now and is not currently capable of causing a pandemic. Only if H5N1 mutates into a form that can be readily transmitted from one person to another could it cause a pandemic.
On August 22, 2007, an Indonesian woman, 28, chicken trader was the 2nd person to die of bird flu on Bali, raising the death toll in the nation due to the disease to 84 (after 4 days of hospitalization). Tests in 2 local laboratories was positive for the H5N1 strain of the disease. 194 people — the majority of them in Indonesia died since 2003, according to the World Health Organization.
On November 16, 2007, newspapers in the U.K. reported that the bird flu crisis had worsened in that nation as a second turkey farm was affected. The Grove Farm in Suffolk was said to have had 20 free range turkeys identified with H5N1.