http://www.pandemicflu.gov/general/whatis.html
U.S. Department of Health and Human Services
A pandemic is a global disease outbreak. An influenza pandemic occurs when a new influenza A virus emerges for which there is little or no immunity in the human population, begins to cause serious illness and then spreads easily person-to-person worldwide. This fact sheet provides the years in which influenza pandemics occurred, lists characteristics and challenges of a pandemic, and discusses how communications and information are critical components of pandemic response.