The monograph addresses the genetics and molecular basis of influenza virus reproduction, its evolution and epidemiology. Influenza virus genome, genome transcription and genomic RNA segments are described. Data on antigenic variation and mutants of influenza viruses, expression of cloned genes, genetic basis of virus virulence are presented. The largest pandemics of the 20th century are covered. For microbiologists, epidemiologists, virologists, geneticists.