What Is the Difference Between a Pandemic and an Epidemic?

"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.

Understanding the difference between an epidemic and a pandemic is helpful 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 explains other terms used to classify the spread of diseases and provides a history of notable outbreaks.

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.

Misuse of 'Epidemic'

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.

Even when "epidemic" 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

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)

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. Tetanus is also a sporadic disease—it only occurs only in unvaccinated individuals.
• 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." This is not an epidemiological term, but one that refers to a contagious bacterial disease characterized by fever and delirium, such as bubonic plague.

Notable  Examples

Endemics and pandemics have occurred throughout human history, including these devastating and noteworthy outbreaks of diseases:

• The Plague of Justinian in 541 A.D. was attributed to the bubonic plague and wiped out 25 to 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 1918 Spanish flu pandemic killed well over 50 million people in one year, including 675,000 Americans.
• The smallpox pandemic of the 20th century claimed between 300 and 500 million lives. In 1980, smallpox was declared eradicated due to a massive campaign launched by the World Health Organization (WHO) in 1959. It is the only human disease that has ever been eradicated.
• The COVID-19 pandemic caused by the SARS-CoV-2 virus claimed nearly 7 million lives worldwide between December 2019 and May 2023. The WHO declared COVID-19 a pandemic on March 11, 2020, prompting shutdowns across the globe. In May 2023, WHO downgraded COVID from a global public health emergency to an ongoing health concern.
• 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.
• The HIV pandemic has killed over 39 million people since 1982. However, with the availability of effective HIV treatment, it is no longer considered a pandemic. It is an epidemic or hyperendemic in specific regions.
• The swine flu pandemic was an outbreak of H1N1 influenza in the United States in 2009. Over 60 million Americans were affected, resulting in 274,304 hospitalizations and 12,469 deaths.
  

Summary

An epidemic is a disease outbreak that is rapidly spreading in a limited region. A pandemic is an epidemic that is actively spreading to multiple regions across the globe.

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, it is commonly referred to as an outbreak. When it is actively spreading or growing out of control, it is an epidemic. Once the disease affects large populations across borders, it is regarded as a pandemic.

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.