Immunofluorescent staining with a monoclonal antibody against influenza A virus was positive, confirming a diagnosis of acute influenza virus infection. Influenza A virus belongs to the Orthomyxoviridae virus family. The viral genome consists of 8 segments of negative single-strand RNA, which collectively encode 10 viral proteins. Influenza viruses are grouped into types on the basis of the nature of the NP, which occurs in one of three antigenic forms, hence types A, B, or C influenza viruses. nfluenza type A viruses are widespread in nature, infecting many avian species, but also humans, pigs, horses, and occasionally other species such as cats. Influenza B virus is an exclusively human pathogen, while influenza C viruses are not serious pathogens in humans. Influenza virus enters via the nasal or oral mucosa. The epidemiology of influenza has several unusual characteristics.