Unlike influenza epidemics which affect the population almost yearly, pandemics occur much less frequently, but have more severe medical and social consequences. The investigation of the nature of the course of all modern epidemics and pandemics are acquiring the particular rationale. Pandemic influenza A (H1N1) 2009 was caused by the virus of the mixed (triple) origin. In Russia, the first three cases of disease have been identified in Moscow from 21 to 10 June 2009. In the Far East - 2-2,5 months later compared to the European part of Russia. However, the epidemic of influenza in Russia caused by influenza virus A (H1N1) pdm09, began and developed more rapidly just in the Far East. The highest morbidity rate (10,2-10,3 per 100 people) was registered in the cities of the Far Eastern and Siberian regions. The phylogenetic analysis allowed to reveal the origin of the triple reassortant virus A (H1N1)pdm09 out of H1N1, H1N2, H3N2 avian/porcine/human virus. The performed analysis of functional domains of proteins of the influenza virus A (H1N1) pdm09 showed that modern pandemic influenza viruses have several principal genetic defects, the totality of which permits to rank them to moderately pathogenic viruses. High risk of the severe course of influenza and occurrence of complications was noted in three groups ofpatients: pregnant women, especially in the 3 trimester ofpregnancy, children under 2 years of age and patients with concomitant chronic respiratory and cardiovascular systems, as well as patients with endocrine disorders and obesity.