Updated unified nomenclature system for the highly pathogenic H5N1 avian influenza viruses World Health Organization (WHO) - October 2011  “In recent years, a variety of different provisional names have been used to refer to emerging lineages of the currently circulating highly pathogenic avian influenza (HPAI) H5N1 viruses. As a result, discussion, comparison, and analysis of the various lineages proved difficult. ... “ See more: