Influenza is an acute respiratory tract infection that can result in a mild in-apparent infection or a severe incapacitating illness. Influenza viruses, the causative agents of influenza, are members of the Orthomyxoviridae; they are further hunched—type A, B, and C—a classification based on genetic differences and antigenic differences among surface proteins and the major internal proteins M and NP. Influenza A viruses are described by a nomenclature this includes the host of origin, geographic origin, strain number, and year of isolation.
Disease caused by influenza virus infection is not a trivial matter. Conceivably, in the United States influenza can kill 10,000 people in a non-pandemic year and 30,000 people after an acute epidemic. Additionally, in terms of physician and hospital expenses and time away from work, the dollar cost to the United States economy is immense. In fact, the Surgeon General of the United States Public Health Service estimated that at least 43 million Americans are at risk of death from influenza by virtue of their underlying medical illness and should be immunized.
Persons over 60 years of age and children less than one year of age have increased case-fatality rates for influenza, probably resulting from decreased immune response with age and complicating cardiopulmonary disease—As the proportion of elderly and children surviving with congenital heart and lung disease increases, so will the need for control of influenza. Additionally, pregnant women, asthmatics, diabetics, and patients receiving immunosuppressive drugs for organ transplants or neo-plastic diseases are at high risk for severe influenza.