The H5N1 virus, popularly called the "bird flu" virus, is a subtype of the Influenza A genus of the Orthomyxoviridae family.
All influenza A subtypes are RNA viruses and H5N1 has a segmented genome of eight single stranded negative sense RNA strands.
H5N1 is also known as A(H5N1), identifying it as an influenza A subtype. The actual type currently being referred to as the bird flu is actually a bird-adapted strain of H5N1. It is highly pathogenic and capable of infecting humans. The specific strain is called HPAI A(H5N1). It is currently infecting millions of birds and has spread over a wide area. The source of the spread is Southeast Asia and there are concerns that a human variant could develop and cause a pandemic much like the Spanish flu in 1918.
H5N1 is being spread mostly by poultry being moved around the world, both alive as well as in the form of poultry derived products. Even infected manure can spread the virus. Humans who have contracted this strain of influenza have been infected by coming in direct contact with poultry. There are other animals that can carry the bird flu virus without themselves becoming sick, some of these animals include ducks, geese and swans. The migrating character of waterfowl is a problem that complicates containing the virus. Viral mixing vessels are of particular concern. An example of a mixing vessels are pigs, who can also carry the virus without becoming ill. The concerning part is not that they carry it, the reason for concern is based on the fact that different strains of influenza could infect the same pig, leading to the possibility of genetic “mixing” possibly producing a strain that could spread among humans. Keeping pigs separated from poultry is therefore a smart precaution to take, but one that is unfortunately not well publicized and especially in developing countries both types of life stock are often kept in close quarters.
H5N1 can spread easily between birds, but not (yet) between humans. The bird flu virus binds to a specific type of galactose receptors present in the avian respiratory tract. These galactose receptors are not present in the human respiratory tract (nose to lungs), and occur only found in alveoli. These are deep inside the lungs and if the virus is bound to them, it cannot easily be released by coughing or sneezing, which would be the most common route of transmission between humans.
