Pandemic - disease outbreak

Pandemic, outbreak of infectious disease that occurs over a wide geographical area and that is of high prevalence, generally affecting a significant proportion of the world’s population, usually over the course of several months. Pandemics arise from epidemics, which are outbreaks of disease confined to one part of the world, such as a single country. Pandemics, especially those involving influenza, sometimes occur in waves, so that a post-pandemic phase, marked by decreased disease activity, may be followed by another period of high disease prevalence.