Occasionally the amount of a disease rises above the normal level for a region or country. This is referred to as an epidemic. The rate of increase in cases can be accelerating, slowing or steady. The word outbreak can also be used to describe a single or group of cases, but epidemic is the more general term and it covers a wider geographical area. A large number of cases across several countries or continents is described as a pandemic.

A pandemic differs from an epidemic in that the disease affects a larger population and more countries. The term is sometimes confused with outbreak and cluster, but these are not the same thing. The difference is that a cluster usually refers to a smaller group of cases, but the WHO definition of pandemic requires an exponential increase in cases over time.

Infectious diseases are responsible for most of the epidemics and pandemics in history. These were often devastating for human society, as was the case with the great smallpox pandemic of 1793 which drove many native peoples into European colonies. Other examples of major premodern pandemics include the introduction of cholera to America and the influenza pandemics in the 1920s and 2030s which led to worldwide economic collapse.

The 21st century has seen the return of old diseases such as cholera and plague, as well as new ones such as SARS and H1N1. Many trends in modern life increase the probability of future pandemics including globalization, urbanization, changing landscapes at the human-animal interface, increased travel and connectivity between populations, climate change and increased interactions between humans (Tyler 2016). Epidemiologic systems must be strengthened at both national and local levels to detect disease, respond quickly and mitigate risk.