What is "pandemic influenza?"

Influenza - "flu" - is widely familiar, causing epidemics every year. Influenza viruses spread very easily from person to person.

A pandemic is an influenza epidemic so large that the entire world is affected. Two main conditions are required for an influenza pandemic: the influenza virus strain must be novel to the human population (so that most of the population does not already have some immunity to it), and it must be able to spread efficiently from person to person.