The term pandemic is defined as "an epidemic that occurs worldwide, or over a very large area, crosses international borders, and usually affects a large number of people." The traditional definition excludes population immunity, virology, and disease severity.Pandemics, according to this definition, occur annually in both the temperate southern and northern hemispheres, because seasonal epidemics cross international borders and affect a large number of people. Seasonal epidemics, on the other hand, are not considered pandemics.