an outbreak of a new flu virus to which people have no immunity and for which there is no vaccine, occurring on multiple continents at the same time

Happens when a new strain of the influenza (flu) virus appears and spreads rapidly in the community. It causes more sickness because there is little or no immunity in the community to a new strain.

Pandemic influenza occurs when a new strain of influenza virus emerges, spreading around the world and infecting many people at once. An influenza virus capable of causing a pandemic is one that people have no natural immunity to, can easily spread from person to person, and is capable of causing severe disease.