What causes flu?

          Flu is caused by the Influenza virus that invades and multiplies in the respiratory tract. Influenza viruses are divided into three types: A, B, and C. This classification is based on the capability of their components to cause disease. Influenza types A and B are the primary causes of the disease and responsible for epidemic outbreaks. Influenza type C is a sporadic cause of predominantly upper respiratory tract disease such as the common cold and pharyngitis.

          The influenza virus is unique in that its structure changes each year. Major changes cause worldwide epidemics called pandemics. Minor changes cause the annual local outbreaks. Pandemics are estimated to occur every 10 to 20 years while local outbreaks occur every 2 to 3 years.

          In the pandemic of 1918 - 1919, the Influenza a virus caused over 20 million deaths worldwide. In 1997, an entirely new variety of human influenza virus surfaced and caused the Hong Kong bird flu in which one-third of the first 18 confirmed cases died.

          Other viruses that known to cause flu-like illness are the respiratory syncyctial virus (RSV) and adenovirus.