Avian influenza viruses (AIVs) are influenza A viruses in the family Orthomyxoviridae that circulate primarily among avian hosts. Influenza A viruses are enveloped viruses with segmented negative-sense single stranded RNA genomes consisting of 8 genomic segments,each encoding one or more viral proteins. The surface proteins of influenza A viruses include 18 known hemagglutinin (HA) and 11 known neuraminidase (NA) types. HA subtypes 1-16 and NA subtypes 1-9 are present in avian influenza viruses, and H17-18 and N10-11 are present in bat influenza viruses. Subtypes A(H1N1) and A(H3N2) circulate seasonally in humans. Lineage and clade classifications are based on the HA, independently of other viral gene segments. Lineages are denoted by geographic location and host species. Clades, which exist within lineages, are assigned numerically and represent genetic differences of >1.5%. Additional branches are added when a single clade evolves into multiple distinct subclades (e.g. 2.3.4.4a and 2.3.4.4b are distinct subclades of clade 2.3.4.4).