Influenza, commonly known as the flu, is a contagious disease caused by a virus infecting the reparatory tract. Influenza viruses are divided into three types, designated as A, B, and C. Influenza type C usually only causes either very mild symptoms or no symptoms at all. Types A and B, however, are responsible for epidemics of respiratory illnesses that can result in hospitalization, or even death. (6)
             Influenza is constantly changing over time by mutations. This continual change allows the virus to evade the antibodies that were meant to kill it. This causes people to be susceptible to influenza infection throughout their entire life. The old antibody may provide partial protection against infection because the viruses are so similar. There are currently four main strains of influenza circulating worldwide. Three of the viruses are type A, and one is type B. Type A viruses are divided into subtypes based on the differences in two viral proteins called hemagglutinin (H) and neuraminidase (N). The three subtypes of influenza A are A(H1N1), A(H3N2), and A(H5N1). The reason for several strands of type A are because of the two kinds of change influenza A undergoes. The first is a series of mutations that occur over time and cause a gradual evolution of the virus, called antigenic "drift." The second known as an antigenic "shift" is an abrupt change in the hemagglutini!
             n and/or the neuraminidase. It is possible for influenza type B to undergo the gradual process of antigenic drift. (1)
             When antigenic shift occurs, large numbers of people have no antibody against the virus. This may result in a worldwide epidemic called a pandemic. Throughout this century there have been three pandemics, which occurred in 1918, 1957, and 1968, each resulting in a large number of deaths. (1) The first and most devastating began in World War I. From 1918 to 1919 the "Spanish flu" caused the highest k...