Avian Influenza (Bird Flu)
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Influenza viruses that infect birds are also called avian influenza viruses. While influenza viruses A and B cause disease in humans, avian flu is caused by type A viruses. Influenza C is rarely identified as a cause of human illness.
There are 15 known avian influenza virus subtypes circulating in bird populations. The severity of the disease depends on the subtype that infects the birds. The illness can sometimes be mild, resulting only in ruffled feathers or reduced egg production, or can be what is known as “highly pathogenic avian influenza.” This latter form is extremely contagious in birds and quickly fatal. Millions of domestic birds have been destroyed in the last few years in an attempt to control the spread of the viruses to animals and humans.