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Bird flu is an infection caused by avian (bird) influenza (flu) viruses. These flu viruses occur naturally among birds. Wild birds worldwide carry the viruses in their intestines, but usually do not get sick from them. However, bird flu is very contagious among birds and can make some domesticated birds, including Chickens, ducks, and turkeys, very sick and kill them. Bird flu viruses do not usually infect humans, but several cases of human infection with bird flu viruses have occurred since 1997.Avian influenza or 'bird flu' is a highly contagious disease of birds, caused by influenza viruses. In birds, the viruses can present with a range of symptoms from mild illness and low mortality to a highly contagious disease with a near 100% fatality rate. The bird flu virus currently affecting poultry and some people in Asia and other areas is the highly pathogenic H5N1 strain of the virus. As the virus can remain viable in contaminated droppings for long periods, it can be spread among birds, and from birds to other animals, through ingestion or inhalation.

There are many different subtypes of influenza a virus. The most virulent are called highly pathogenic avian influenza and can reach epidemic levels among birds.

Of these, subtype H5, and more particularly subtype H5N1 currently, pose the greatest concern for human health. Two other subtypes - H9 and H7 - have caused illness in people but neither has caused outbreaks in poultry as severe as H5N1-related ones.

The H5N1 virus does not usually infect humans. In 1997, however, the first case of spread from a bird to a human was seen during an outbreak of bird flu in poultry in Hong Kong. There have been other cases of H5N1 infection among humans. Most recently, human cases of H5N1 infection have occurred in Thailand, Vietnam and Cambodia during large H5N1 outbreaks in poultry. The death rate for these reported cases has been about 50 percent. Most of these cases occurred from contact with infected poultry or contaminated surfaces; however, it is thought that a few cases of human-to-human spread of H5N1 have occurred. avian influenza

Bird flu infect people
H5N1 is able to infect people because it is able to cross the species-barrier, although it does not do this easily. In human populations, where domestic pigs and wild and domestic birds live in close proximity with people, the mingling and exchange of human and animal viruses can more easily occur. Those who have become infected have had close direct contact with infected birds.

Human infection with avian influenza viruses usually causes mild conditions such as conjunctivitis (eye infection) and mild flu-like symptoms, with one notable exception, the highly pathogenic H5N1 virus. More severe infection can lead to pneumonia, acute respiratory distress, viral pneumonia, and other severe and life-threatening complications.

Infection
The causative agent is the avian influenza (AI) virus. AI viruses all belong to the influenza virus a genus of the Orthomyxoviridae family and are negative-stranded, segmented RNA viruses Avian influenza spreads in the air and in manure. Wild fowl often act as resistant carriers, spreading it to more susceptible domestic stocks. It can also be transmitted by contaminated feed, water, equipment and clothing; however, there is no evidence that the virus can survive in well cooked meat. The incubation period is 3 to 5 days. Symptoms in animals vary, but virulent strains can cause death within a few days.