Suatu penyakit menular yang disebabkan oleh virus influenza tipe A dan ditularkan oleh unggas. Penyakit flu burung yang disebabkan oleh virus avian infuenza sub tipe H5N1 pada unggas dikonfirmasikan telah terjadi di Republik Korea, Vietnam, Jepang, Thailand, Kamboja, Taiwan, Laos, China, Indonesia dan Pakistan. Sumber virus diduga berasal dari migrasi burung dan transportasi unggas yang terinfeksi.
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"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. Out of the three types of influenza viruses (A, B, and C), influenza A virus is a zoonotic infection with a natural reservoir almost entirely in birds. Avian influenza, for most purposes, refers to the influenza A virus.
Though influenza A is adapted to birds, it can also stably adapt and sustain person-to person transmission. Recent influenza research into the genes of the Spanish flu virus shows it to have genes adapted from both human and avian strains. Pigs can also be infected with human, avian, and swine influenza viruses, allow for mixtures of genes (reassortment) to create a new virus, which can cause an antigenic shift to a new influenza A virus subtype which most people have little to no immune protection.
Avian influenza strains are divided into two types based on their pathogenicity: high pathogenicity (HP) or low pathogenicity (LP). The most well-known HPAI strain, H5N1, appeared in China in 1996, and also has low pathogenic strains found in North America. Companion birds in captivity are unlikely to contract the virus and there has been no report of a companion bird with avian influenza since 2003. Pigeons do not contract or spread the virus.