Today, acute respiratory infection (ARI) and influenza occupy the first place among the infectious diseases transmitted by airborne droplets. Influenza viruses are well adapted to animals and humans, affecting 5 to 30% of the world population, which leads to the death of people and animals, causing enormous economic damage. Influenza A(H1N1)pdn09 virus caused a pandemic in 2009. It is still largely unknown the molecular mechanism by which A(H1N1)pdm09 viruses were able to adapt to animals and then to humans. To address this problem, we obtained three adapted variants of pandemic A(H1N1)pdm09 virus by serial lungto-lung passages in different genotypes of mice. The cross-infection of A(H1N1)pdm09 viruses helps to evaluate the virulence of pathogens. Were obtained three adapted variants of the influenza A(H1N1)pdm09 virus. The most hight virulence possess the strain A/Tomsk/273-MA1/2010(H1N1pdm09) which were adapted to BALB/c mice
