Epidemic vs. Pandemic: What Are the Differences?

Epidemics are sudden disease spreads; pandemics are epidemics that spread widely

"Epidemic" and "pandemic" are two words that describe the spread of disease. "Epidemic" is used to describe a disease that has grown out of control and is actively spreading. "Pandemic" is used to describe a disease that affects a whole country or the entire world.

It's important to know the difference between an epidemic and a pandemic, especially when considering public health news. Terms like these are meant to help the public respond to a health crisis so they can better control and prevent disease.

This article discusses the difference between the terms "epidemic" and "pandemic." It also covers how experts classify diseases based on where the disease has spread and how many people are affected, along with a list of notable pandemics in history.

Epidemics vs. Pandemics
Verywell / JR Bee

Epidemic vs. Pandemic

Medical professionals use the terms "epidemic" and "pandemic" to distinguish between the size and scale of a disease's spread. When you hear the word "epidemic," it generally refers to a sudden increase in the number of cases of a disease above what is normally expected. 

For example, from 2014 to 2016, Ebola was considered an epidemic because the disease was spreading rapidly throughout parts of West Africa but did not spread throughout other parts of the world. 

A pandemic, however, refers to an epidemic that has spread over several countries or continents, usually affecting a large number of people. COVID-19 is a pandemic because millions of cases have occurred all over the world.

What Is "Endemic"?

"Epidemic" is sometimes confused with "endemic," but the two words have different meanings. While "epidemic" describes how far a specific outbreak of a disease has spread, the word "endemic" refers to the constant presence of a disease in a geographic population.

For example, chicken pox is considered endemic in the United States because it affects American school children at predictable rates.

Common Confusion

The term "epidemic" is used in a couple of different ways, mainly to describe:

  • Matters of health, for example: The opioid crisis in America has grown to epidemic proportions.
  • Behavior, for example: There's an epidemic of tantrums among preschoolers!

These usages are not wrong, but they can cause confusion. Also, even when the word is used to define health issues, it may not accurately describe the scale of the disease or how quickly it is spreading.

In some cases, "epidemic" may fall short in describing the scale of the problem, and the word "pandemic" may be more fitting.

Disease Event Classification

According to the reference "Epidemiology for the Uninitiated," epidemiology is the branch of medicine that studies how often diseases occur in different groups of people and why.

In the United States, the Centers for Disease Control and Prevention (CDC) is the main body that collects and oversees epidemiological data. Among its many functions, the CDC is tasked with directing the appropriate response to a disease occurrence.

While the level of disease occurrence can be described in many ways, it is primarily defined by two measurable factors:

  • The pattern and speed by which a disease moves (known as the reproduction rate)
  • The size of the at-risk population (known as the critical community size)

The role of epidemiology is to determine the disease prevalence (how many people within a population have the disease) and incidence (the number of new cases within a certain timeframe). These figures help direct the appropriate public health response.

Other Definitions

There are several other ways an epidemiologist might describe a disease event:

  • Sporadic refers to a disease that occurs irregularly or infrequently. Foodborne pathogens, such as Salmonella or E. coli, can often cause sporadic disease outbreaks.
  • Cluster refers to a disease that occurs in larger numbers even though the actual number or cause may be uncertain. An example is the cluster of cancer cases often reported after a chemical or nuclear plant disaster.
  • Hyperendemic refers to persistent, high levels of disease well above what is seen in other populations. For example, HIV is hyperendemic in parts of Africa, where as many as one in five adults has the disease, in contrast to the United States, where roughly one in 300 is infected.
  • Outbreak carries the same definition as an epidemic but is often used to describe an event that is more limited to a geographic area.

You may have heard the term plague before, too. This is not an epidemiological term, but one that refers to a contagious bacterial disease characterized by fever and delirium, such as bubonic plague.

Word Choice

The distinction between the terms "outbreak," "epidemic," and "pandemic" is often blurred, even among epidemiologists.

Part of the reason for this is that some diseases become more widespread or lethal over time, while others become less, forcing the CDC to adjust the models it uses to describe them.

Epidemiologists have to be cautious about how they describe a disease event so that the public is well-informed about how to respond.

On one hand, labeling the disease as less of a risk than it actually is could prevent people from protecting themselves. On the other, labeling the disease as a bigger risk than it actually is could incite more panic than necessary.

One such example is the Zika outbreak of 2016, which triggered alarm in the United States when 218 people in Florida and six people in Texas became infected. Another 46 were infected from sexual or laboratory transmission, and one additional person became infected from person-to-person contact through an unknown route.

The 2022 mpox (formerly known as monkeypox) outbreak is another example. Though it spread throughout the world, it was not labeled an epidemic or a pandemic because the virus is not easily transmittable among people who are not in close contact.

Even with HIV, a disease spread across much of the planet, the term "pandemic" has been increasingly replaced by "epidemic." This is because effective HIV treatment is widely available, and as a result, rates of the disease are decreasing in regions where it was previously hyper-prevalent.

On the other hand, as influenza becomes more severe year after year, public health officials will commonly refer to the seasonal outbreaks as pandemics, particularly given the 2009 H1N1 outbreak in the United States, in which over 60 million Americans were affected, resulting in 274,304 hospitalizations and 12,469 deaths.

This is not to suggest that pandemics are treated the same as more contained outbreaks, in part because pandemics often require officials in multiple countries to work together. At the same time, outbreaks like the Ebola virus, which have the potential to expand beyond borders, need to be treated as aggressively as a pandemic.


Whereas an outbreak usually refers to a disease that has broken out in a limited region, a pandemic is one that affects large numbers of people—typically across the world. An epidemic falls between the two; it is an outbreak that is actively spreading and may have potential to become a pandemic.

Phases of a Pandemic

The CDC has procedures to evaluate and classify a disease event. Still, the actual staging of an epidemic, which outlines when the disease spread is severe enough to take specific actions, can vary based on the pathogenesis (pathway) of a disease and numerous other factors.

Pathogenesis is the step-by-step process by which an infection becomes a disease in the body. It includes how a person gets infected, for example, by skin-to-skin contact, along with which organs the disease targets, and how the disease is shed in the environment, such as by lingering in the air or sticking to surfaces.

The one staging model used to direct the public health response specifically involves influenza (the flu). In 1999, the World Health Organization (WHO) released the very first influenza pandemic preparedness plan, which outlined the appropriate response based on six phases.

The aim of the plan was to provide countries with a blueprint from which to draw up their own national strategies based on available resources. The United States released its first pandemic influenza plan in 2005. The same basic model can be applied with variations to other epidemics, such as tuberculosismalaria, and the Zika virus.

Phases 1 through 3 are designed to help public health officials know it is time to develop the tools and action plans to respond to a new threat. Phases 4 through 6 are when action plans are implemented in coordination with the WHO.

The WHO revised the phases in 2009 to better distinguish between preparedness and response. The plan was meant to address influenza pandemics given their high mutation rate and the virus's ability to jump from animals to humans.

Former WHO Stages of a Flu Pandemic

  • Phase 1 is the period during which no animal viruses are reported to cause infection in humans.
  • Phase 2 is the first level of threat wherein a virus is confirmed to have jumped from an animal to humans.
  • Phase 3 is when sporadic cases or small clusters of disease are confirmed, but human-to-human transmission has either not occurred or is considered unlikely to sustain an outbreak.
  • Phase 4 is the point where either human-to-human transmission or a human-animal virus has caused a community-wide outbreak.
  • Phase 5 is when human-to-human transmission of the virus has caused the spread of disease to at least two countries.
  • Phase 6 is the point at which the disease is declared a pandemic, having spread to at least one other country.

The timeframe for each phase can vary significantly, ranging from months to decades. Not all will progress to phase 6, and some may even revert if a virus weakens.

WHO stopped using this six-step plan in February of 2020.

Notable Pandemics in History

In addition to HIV, which has killed over 39 million people since 1982, there have been other equally devastating pandemics in history:

  • The Plague of Justinian of 541 A.D. was attributed to the bubonic plague and wiped out 25-50 million people in one year.
  • The Black plague killed more than 75 million people from 1347 to 1351, including those who died in Europe, Middle Eastern lands, China, and India.
  • The Spanish flu pandemic of 1918 killed well over 50 million people in one year, including 675,000 Americans.
  • The smallpox pandemic of the 20th century claimed between 300 to 500 million lives. In 1980, smallpox was declared eradicated due to a massive campaign launched by WHO in 1959. It is the only human disease that has ever been eradicated.
  • COVID-19 was first detected in China in December of 2019. By early 2020, the SARS-CoV-2 virus was spreading throughout Europe and North America. The World Health Organization (WHO) declared COVID-19 a pandemic on March 11, 2020. In its first two years, the COVID-19 pandemic claimed more than 6 million lives worldwide.
  • The tuberculosis pandemic continues to kill over 1.5 million people annually. Despite the availability of effective treatment, the bacteria that causes tuberculosis is increasingly resistant to drugs used to treat it.


Epidemiologists use staging models to direct public health responses and help leaders work together to stop disease progression. The first staging model was created in response to influenza, and different models are used for different diseases since disease pathogenesis varies.


Epidemiologists are experts in disease progression. When a disease event occurs, they help direct the public health response by classifying how big a threat the disease is.

If the disease is limited to an isolated region, epidemiologists may refer to it as an outbreak. When it is actively spreading or growing out of control, they may refer to it as an epidemic. Once the disease affects large populations across borders, it is regarded as a pandemic.

To stop a disease from progressing from an outbreak to a full-blown pandemic, the WHO and the CDC use staging models with multiple phases, which help experts and world leaders coordinate a response using the resources available to them.

Frequently Asked Questions

  • What is a sporadic disease?

    A disease is considered sporadic if it occurs infrequently or irregularly; in other words, it is difficult to determine when it might appear again. Tetanus is considered a sporadic disease that occurs only in those who have not received tetanus vaccinations. A few other sporadic diseases include E. coli, salmonella, and plague.

  • What is influenza?

    Influenza, more commonly called the flu, is a virus that infects the nose, throat, and lungs. It is contagious, usually spread by tiny droplets that spread from one person to another, and causes seasonal flu epidemics. Common symptoms include fever, cough, sore throat, fatigue, muscle aches, runny or stuffy nose, and headaches.

  • What does virulent mean?

    When an infection or disease is virulent, it is considered capable of causing severe illness. The word has been used in reference to different pandemics and epidemics throughout history.

A Word From Verywell

Feelings of fear, anxiety, sadness, and uncertainty are normal during pandemics. Being proactive about your mental health can help to keep both your mind and body stronger. Learn about the best online therapy options available to you.

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.

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Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Trisha Torrey
 Trisha Torrey is a patient empowerment and advocacy consultant. She has written several books about patient advocacy and how to best navigate the healthcare system.