Bird flu, also called avian flu or Asian bird flu, is caused by a virus that normally only infects birds (and in rare cases pigs). The bird flu is a strain of influenza virus known as H5N1.
The first historical cases of avian flu were identified in Europe, specifically Italian chickens, in the 1870s. The Oxford Dictionary states the origin of the word "influenza" to be Italy in 1743, the location of the first reported epidemic according to CBC (Canada Broadcasting Company). The virus has infected humans and animals for centuries. The first cases of bird flu in recent years were documented in Hong Kong (according to avianinfluenza.org). Another outbreak occurred there in 2003. As infected wild birds migrate they share the virus with domestic populations in other areas of the world as well as native wild birds in those regions. These birds then can spread the virus among the human inhabitants. The first human cases were documented in the open farm markets of Vietnam, China, and Indonesia in 2005. In 2006 a case was reported in the Middle East. The presence of the virus in ducks in Nigeria show that is has spread even further. Some articles dispute that migratory pathways are to blame, even though that is the mechanism proposed by the World Health Organization, and state that airplane travel allows human carriers to spread the disease. Although this may be true, those people would not be capable of passing the virus to other people and the spread would not continue. The migration theory is supported by the die-off of 6000 migratory birds beginning at the Qinghai Lake nature reserve in central China in late April 2005. There is also accumulating evidence that the virus follows trade routes, which would most likely be in birds being transported for sale.
The origin of human infection with bird flu is thought to be via bird droppings. The origins of the strain itself and its presence in birds is probably a similar strain of influenza from which it mutated. Ingestion or fluid exchange may be introducing a new route of transmission for this strain as opposed to the familiar aerosol, or airborne, transmission influenza most commonly uses. A Chinese father who contracted bird flu from his son, was found to have been in contact with 91 people, none of which contracted the disease. Genetic analysis (published by Lancet) has found that the bird flu strain has not undergone any major mutation that would allow it to spread more readily among humans.
There are a few widely sensationalized cases of spread among family members, but the bird flu H5N1 strain, as of now, has shown a lack of ability to bind to the cells of the human respiratory tract, which is necessary for the virus to infect humans and limits its spread. Mutations in the virus's ability to adhere to cells in the human respiratory system would make it a formidable opponent for modern medicine.