Influenza viruses are responsible for epidemics of respiratory illness that occur every winter and are often associated with increased rates of hospitalization and death. While most people who get the flu usually recover completely in one to two weeks, some people develop serious and potentially life-threatening medical complications such as pneumonia. Although flu-related complications can occur at any age, the elderly and persons with chronic health problems are much more likely to develop serious complications.

Influenza viruses continually change over time, usually by mutation. This constant changing enables the virus to evade the immune system of its host, so that people are susceptible to influenza virus infection throughout life. A person infected in a given year with the current influenza virus develops antibody against that virus. As the virus changes over time, the "older" antibody no longer recognizes the "newer" virus, and in subsequent years reinfection can occur.

Influenza viruses undergo two kinds of changes. One is a series of mutations that occur over time and cause a gradual evolution of the virus. This is called antigenic "drift." The other kind of change is an abrupt change in specific viral proteins. This is called antigenic "shift." In this case, a new subtype of the virus suddenly emerges, with potentially devastating consequences. New deadly strains of flu appear every few years and recent reports suggest an impending threat from extremely virulent strains derived from birds in Southeast Asia – the “Bird Flu” – that has already killed several people in Hong Kong and Vietnam.