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In this review of literature modern notions on the role of birds in the evolution of the pathogenicity signs and immune system of the main and intermediate hosts of influenza viruses, as well as on the mechanisms of overcoming interspecific barriers, are analyzed. The chronology of the spread of "avian" influenza among humans, starting from 1997, the properties of the natural reservoir of this infection, and in particular influenza viruses A, the ways of their variability and evolution are presented. The conclusion has been made that the mixing, joint evolution, recombination and reassortment of viral genomes may be caused by global events in individual geographical regions.

In the period of 1993 to 1994, influenza activity caused by circulation of viruses antigenically structurally related to the strain A/Beijing/32/92 (H3N2) began in the countries of North America and West Europe in October-November 1993, by spreading to the countries of East Europe, the Urals, and West Siberia. Epidemic events in China and in the Far East were evoked mainly by the influenza B virus. The influenza A (H1N1) viruses did not widely spread. Single isolates (about 1% of all the influenza A virus isolates, as evidenced by the CDC, Atlanta, USA) of this subtype were etiological agents of sporadic morbidity. A fourteen-year period (1977-1991) of the epidemic activity of the influenza A (H1N1) virus seems to be over. The last epidemic of the influenza A (H3N2) virus has a moderate intensity. Among all the age-group populations, children were largely afflicted. The highest incidence in Russia was recorded in Arkhangelsk, Barnaul, Nizhny Novgorod, Omsk, Samara, and Smolensk where approximately 16-18% of children under 14 years of age and 3.5-5% of the population of 15 years or more fell ill. The highest incidence was recorded in Minsk and Vilnus too.