Influenza, which affects people and animals, is caused by type A influenza viruses, a member of the Orthomyxoviridae family. The viruses are characterized by a considerable antigen variability. Two kinds of the variability are known: antigenic shift and antigenic drift. Type A viruses appear in populations of birds worldwide. They may be pathogenic for some birds and non-pathogenic for others. Infected wild water fowl, migratory and marine birds which show no clinical signs are the largest natural reservoir for all types of influenza viruses and they are the main source of infection for domestic birds and other animals. The influenza is zoonosis. H5N1 influenza virus strain was initially transmitted directly to humans in Asia in 1997. The virus has subsequently appeared at the end of 2003 in Hong Kong and it has been diffused to several countries of Asia. According to WHO, the total number of affected persons in these countries on December 07, 2005 was 137 of which 70 of them died. In 2005 avian influenza was noticed in Russia, Kazakhstan, Mongolia, Romania and Turkey. In these countries no infections in humans were observed. According to WHO, FAO and OIE the avian influenza appearing in many Asian countries and expanding to the West is a serious threat for human health.
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