IntroductionThe last major influenza virus epidemic occurred in 1918, when the H1N1 "Spanish Flu" pandemic killed between 20 and 40 million people worldwide. Before then, a pandemic would occur about once a century. However, this influenza pandemic was unusual because of the high number of deaths among healthy, young people, the speed with which it spread, and its lethality. Although vaccines had already been developed and distributed to the public, the search for a cure for the influenza virus was delayed because scientists believed that the flu was caused by the bacterium Haemophilius Influenzae. In 1933, the deadly disease was discovered to be caused by a virus and research was underway for a vaccine. This was accomplished soon after, when Dr. Jonas Salk and Thomas Francis developed the first inactivated influenza vaccine in 1938 using the same egg-based methods already established for producing vaccines. The vaccine was initially used for soldiers during World War II. Unfortunately, the first vaccines were not very effective, since it was almost impossible to measure the potency of the va...