Timeline of the Biggest Pandemics in History

Introduction

A pandemic (from Greek πᾶν, pan, "all" and δῆμος, demos, "local people" the 'crowd') is an epidemic of an infectious disease that has spread across a large region, for instance multiple continents or worldwide, affecting a substantial number of people. A widespread endemic disease with a stable number of infected people is not a pandemic

A pandemic is the global outbreak of a disease. When an epidemic spreads beyond a country’s borders, that’s when the disease officially becomes a pandemic. Pandemics are generally classified as epidemics first, which is the rapid spread of a disease across a particular region or regions. But epidemics don't always become pandemics, and it's not always a fast or clear transition.

The Centers for Disease Control and Prevention (CDC) defines an epidemic as being when there’s an increase –often suddenly– in the number of cases of an infectious disease above what is normally expected in a localized population. It defines a pandemic as an epidemic that has spread over several countries or even continents. Pandemics usually affect large segments of the population.