An epidemic caused by a new virus, for which people have no immunity,  spreads easily from person to person, and crosses international boundaries usually affecting a  large number of people is called a “pandemic.” Pandemics can spread rapidly around the world in only a few months. The current Bird Flu virus does not spread easily between people now, but it could change (mutate) into a form that spreads easily from person to person.  No vaccines are available now, because a vaccine cannot be developed until the human pandemic flu virus exists.