Influenza A and B are the two types of influenza viruses that cause epidemic human disease. Influenza type C infections cause a mild respiratory illness and are not thought to cause epidemics.Influenza A viruses are further categorized into subtypes on the basis of two surface antigens: hemagglutinin (H) and neuraminidase (N). Strains are also described by geographic origin, strain number and year of isolation.详情请登陆EterLife官网:www.eterlife.com