What is epidemic? 

　　The occurrence in a community or region of cases of influenza clearly in excess of normal year-round frequency.

　　What is pandemic? 

　　In general, a pandemic is an epidemic occurring over a very wide area (at least a whole continent) and usually affecting a large portion of the population. According to this definition, there is an influenza pandemic each year. With influenza, however, the term pandemic is restricted to a worldwide epidemic caused by a new influenza virus subtype. Such a new subtype is characterized by major changes in protein antigens found on the surface of the influenza virus particle. As most people have not had the chance to become immune to these new strains, widespread infection may occur. Pandemics only occur with the influenza A virus.

　　When is the next pandemic likely to occur? 

　　It is not known. In the 20th century, severe pandemics occurred in 1918 ("Spanish Flu"), 1957 ("Asian Flu") and 1968 ("Hong Kong Flu").