A Brief Review of the Epidemiology of Influenza and Recent Advances in the Study of the Virus

Epidemics of influenza have occurred, so far as is definitely known, for the past 400 years, and possibly hundreds of years before that. Three distinct types of influenza virus have been isolated and described to date. The first was labeled type A. A second type, designated as influenza virus B, was isolated independently by Francis and by Magill in 1940. A third type of influenza virus, type C, was found in 1947. More recently both influenza types A and B have been divided into subtypes such as A1 and A2, B1, B2 and B3. Influenza type C virus exists as a single and stable antigenic type. It is believed that influenza vaccination is effective for only up to three months under the best of circumstances because influenza is essentially an external disease. Since the area affected is predominantly the upper respiratory tract, a much smaller quantity of antibody reaches these cells than is found in the blood. Some of the more recent knowledge relating to the physical, biochemical and immunological characteristics of the influenza viruses, and how these characteristics are used in the most recent classification of the myxoviruses are discussed.