Influenza: propagation, quantification, and storage.

Influenza viruses are negative-sense, single-stranded, enveloped RNA viruses belonging to the family Orthomyxoviridae. Three types exist, influenza A, B, and C. All infect humans, but only A and B are major human pathogens. Influenza type A viruses are divided into subtypes based on genetic and antigenic differences in the two surface spike proteins, hemagglutinin (HA) and neuraminidase (NA). The appropriate cell lines to be used for isolation of influenza A or B viruses depend on the clinical information and the host of origin. MDCK cells are the preferred cell line for isolation of human influenza viruses from clinical specimens.