Antigenic shift usually occurs in animal hosts and is responsible for producing both epidemics and pandemics. Orthomyxoviruses contain a single stranded, negative RNA genome divided into 8 segments. The H and N antigens are variable. There are about 13 different H antigens and 9 different N antigens found in birds. There are only about 3 combinations that affect humans, however.
Viral attachment is mediated by the hemagglutinin.
The virus enters host cells by pinocytosis and uncoating occurs by fusion of the viral envelope with the membrane of the vacuole. The RNA is capped and replication proceeds in the nucleus. The progeny are released by budding and cell death ensues. The segmented genome of the influenza virus allows rearrangements to occur in simultaneously infected cells. This accounts for the periodic appearance of new variants.