Influenza, or "the flu" is a highly contagious viral infection, with the disease affecting hundreds of thousands of people every year. Most people protect themselves from the flu by receiving a flu shot or the flu mist every fall. The ability for the flu virus to "drift", or make slight structural changes over time results in the appearance of the different strains which circulate in the human population each year. Scientists are able to study these "drifts" and develop a vaccine each year to match the strains which are expected.
By contrast to this gradual and expected process, the flu virus can change suddenly and dramatically. In this type of quick change, a new or "novel" influenza virus rises and spreads rapidly over the entire world. Very few members of the population at large have a natural immunity to fight off this new virus. Pandemic Influenza is a worldwide outbreak of influenza in humans. In order to be considered "Pandemic" or "Worldwide" the outbreak must be:
 A strain of the flu which most of the population of the world does not have a natural immunity against - meaning their body is not capable of fighting off the virus without help
Cause serious, often fatal, illness
Have the ability to be spread by human to human quickly
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