What is the difference between pandemic influenza and seasonal influenza?
Seasonal outbreaks (epidemics) are caused by subtypes of influenza viruses that are already in existence among people, whereas pandemic outbreaks are caused by new subtypes or by subtypes that have never circulated (spread) among people or that have not circulated among people for a long time. Past influenza pandemics have led to high levels of illness, death, social disruption, and economic loss.
