[The definition of pandemic: causes and consequences of its adjustment in 2009 and 2011].

The article considers results of the study evaluating historical and epidemiological events that preceded and accompanied adjustment of the pandemic description on the WHO website in 2009 and 2011. The analysis covered publications related to epidemics and pandemics issues, the WHO documents, the WHO website. The descriptions of pandemic mostly focused on "enormous numbers of cases and deaths". Since May 2009, new description of pandemic was published, focusing on disease prevalence. In 2011 it reverted to initial one with no comments. From perspective of the WHO document of 2009, declaration of swine flu pandemic in June 2009 seemed justified. However, considering previous pandemic history, common sense and consequences of declaring pandemic of disease with low both number of cases and mortality, it was premature move. Since primary factor hindering development of pandemic is effectiveness of infectious disease treatment, to minimize likelihood of new pandemic it is necessary to improve special medical education quality and to study and to adapt to modern conditions all effective medications and methods used in the past.