A Risk Sciences International glossary definition (Last modifed: October 22, 2023)


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A pandemic is defined as an outbreak of a disease that occurs over a wide geographic area and affects an exceptionally high proportion of the population, often crossing international borders. Pandemics are characterized by the widespread, usually global, distribution of a novel infectious agent to which the general population has little to no immunity. The consequences of a pandemic are generally severe, affecting public health systems, economies, and social structures on a global scale. Examples of pandemics include the H1N1 influenza in 2009 and the COVID-19 outbreak that started in 2019.

An epidemic, by contrast, is a sudden increase in the number of cases of a particular disease above what is normally expected in a specific geographic area or among a set population. Epidemics are localized events, even if the term is sometimes used to describe a sudden spike in cases on a national level. The causative agents in an epidemic are often already known, and the affected population may have some level of pre-existing immunity. Therefore, the scope, both in terms of geography and impact, is generally more limited compared to a pandemic.

The primary differences between an epidemic and a pandemic are as follows:

  1. Geographical Scope: An epidemic is generally confined to a community, region, or country, while a pandemic has a global reach.
  2. Population Affected: Epidemics usually affect a smaller proportion of the population compared to pandemics, which can impact entire nations or continents.
  3. Immunity: In a pandemic, the general population usually has little to no pre-existing immunity to the infectious agent, while in an epidemic, there is often some level of immunity or a previous understanding of how to manage the disease.
  4. Duration: Epidemics are often shorter-lived, as they are generally easier to control given their limited geographic range. Pandemics can last for extended periods, affecting multiple waves of populations as the disease spreads.
  5. Impact: Pandemics often have broader social, economic, and political consequences compared to epidemics due to their larger scale and longer duration.

In summary, while both pandemics and epidemics refer to the spread of infectious diseases, they differ in terms of geographical scope, population affected, immunity, duration, and overall impact. A pandemic is a global event affecting a vast number of people and generally having far-reaching implications, whereas an epidemic is a more localized event, usually confined to a specific geographic area or population.

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