| characterization in vitro and in vivo of pandemic (h1n1) 2009 influenza viruses isolated from patients. | the first influenza pandemic of the 21st century was caused by novel h1n1 viruses that emerged in early 2009. molecular evolutionary analyses of the 2009 pandemic influenza a h1n1 [a(h1n1)pdm09] virus revealed two major clusters, cluster i and cluster ii. although the pathogenicity of viruses belonging to cluster i, which became extinct by the end of 2009, has been examined in a nonhuman primate model, the pathogenic potential of viruses belonging to cluster ii, which has spread more widely in t ... | 2012 | 22718834 |