[Classification and nomenclature of influenza viruses]

Criteria for classifying influenza viruses into genera, types and subtypes have been discussed, with reference to the 6th report of ICTC. The viruses are divided into 2 genera based on the number of the genome RNA segments. Those which contain 8 segments are further divided into types A or B, according to the antigenicity of the NP and M1 proteins. Viruses belonging to type A are finally classified into subtypes according to the serotype of the HA and NA proteins. Thirteen and nine serotypes of HA and NA, respectively, have been identified in nature, where a variety of combination is possible by genetic reassortment. Method for nomenclature of influenza viruses has also been discussed.