http://pandemicflu.gov/general/
U.S. Department of Health and Human Services
A flu pandemic occurs when a new influenza virus emerges for which people have little or no immunity and for which there is no vaccine. The disease spreads easily person-to-person, causes serious illness, and can sweep across the country and around the world in very short time. This document answers questions about pandemic flu, including is there a vaccine and what would be the impact of a pandemic.