• Brittany Evans

What Classifies an Outbreak, Epidemic, or Pandemic?

“Pandemic” is a word that the world became familiar with during the COVID-19 Pandemic of 2020. Similar words like outbreak, epidemic, and endemic have been used interchangeably to describe viruses and diseases in the past. During the influenza pandemic of 1789-1790, pandemic and epidemic were used interchangeably. Webster's dictionary even went so far as to list pandemic and epidemic as synonymous words. Even in recent years, the definitions of these words have been questioned. However, in 2009, the WHO changed a statement that was on their pandemic preparedness homepage which appeared to define the word “pandemic.” The change took place during the H1N1 pandemic of 2009 and caused an uproar amongst the public and health officials. The Council of Europe, in particular, felt that the change made it easier for the WHO to declare a pandemic. After all of the definitions and descriptions of these words, it leaves the public wondering what these words mean concerning public health?

Let's start small, an endemic is defined as a constant presence of a disease or infectious agent in a population within a geographic area. For example, Malaria, a disease that is transmitted through mosquito bites, is endemic in Nigeria, Ethiopia, and Zimbabwe to name a few locations. Endemic diseases can quickly become outbreaks, just one case outside of the endemic zone is considered an outbreak because it breaks the "zero-endemic level". Plainly stated, this means that simply one case of malaria in the United States is considered an outbreak. Outbreaks are restricted to small areas, such as the cholera outbreak following the 2010 Haiti earthquake. The problem with the word “outbreak” is that it has many meanings depending on the context.

The Journal of Public Health Policy*(paywall) noted that there are at least six varying definitions of the word outbreak. Each definition has a base meaning that builds into a new context. The problem with multiple definitions for different contexts is the fact that it can blur the lines for public health issues. If each public health organization follows a different version of the definition, it can cause public fear and confusion. Without a clear definition of the term “outbreak,” the word can continue to imply many things. Having a firm definition in place for each term will help public health officials and the public respond more appropriately to each situation. Consider for a second the depth of the terms endemic, outbreak, epidemic, and pandemic. These are emotional terms that can strike unwelcome fear into communities when used improperly. A more uniform usage and restricted meaning of the terms is a critical step in understanding disease.

An epidemic can be defined as an outbreak that spreads over a large geographical area. One example of an epidemic is the U.S. opioid epidemic. In the late 1990s, pharmaceutical companies reassured the medical community that patients would not become addicted to prescription opioid pain relievers, and healthcare providers began to prescribe them at greater rates. This subsequently led to widespread diversion and misuse of these medications before it became clear that these medications could indeed be highly addictive. The crisis has steadily gained power and increased its death toll. In 2017, the U.S. Department of Health and Human Services declared the opioid crisis a public health emergency.

In turn, the largest of the outbreaks is considered a pandemic. The International Epidemiology Association’s Dictionary of Epidemiology defines a pandemic as an epidemic occurring worldwide, or over a very wide area, crossing international boundaries and usually affecting a large number of people. An easy way to remember the difference is that a pandemic is considered an epidemic with a passport, plainly stating that a pandemic has a larger geographical range. Much like outbreaks, there is no set definition of the word pandemic as it applies to public health. The Journal of Infectious Diseases lists eight key features that should apply to all or almost all of the diseases before we can classify a pandemic. The Journal lists wide geographic extension, disease movement, high attack rates and explosiveness, minimal population immunity, novelty, infectiousness, contagiousness, and severity as key factors in determining whether an outbreak or epidemic should be considered a pandemic. Knowing how difficult it is to fit a disease into all eight categories could explain why there is some overlap in the use of these terms.

How can the general public determine the difference between an outbreak, endemic, epidemic, and pandemic? There is no easy way to determine what scenario is unfolding without all the proper information. As a member of the public, it can sometimes be difficult to find the in-depth information needed to make a clear decision. What someone can do is follow the guidelines put in place by the CDC, the WHO, and your local health and human services departments. If anyone is ever unsure of a scenario and the effects it has on one’s community, reach out to a local health department or health official with questions.


Speaking Plainly:

  • Outbreaks, endemics, epidemics, and pandemics are all words used to describe the spread of disease and the varying degrees of the spread.

  • The terms do not always have a clear definition and are sometimes abused or used incorrectly which can cause fear and concern in the community.

  • If anyone is ever uncertain about a rising health situation in one’s community, reach out to the local health department or health officials for help understanding the situation.