On March 11, 2020, the Director-General of the World Health Organization (WHO) declared the international spread of a new coronavirus, SARS-CoV-2, a worldwide pandemic.

Some news organizations and public health officials had been calling the outbreak a pandemic weeks earlier than the WHO declaration — so how do you know when an outbreak becomes an epidemic and an epidemic becomes a pandemic?

Though public health definitions shift and evolve over time, the distinctions between these terms are generally a matter of scale. In short, a pandemic is an epidemic that has gone global.

The Centers for Disease Control and Prevention (CDC) defines an epidemic as an unexpected increase in the number of disease cases in a specific geographical area.

An epidemic is any rise in cases beyond the baseline for that geographic area.

Epidemics can occur:

  • when an infectious agent (such as a virus) suddenly becomes much more prevalent in an area where it already existed
  • when an outbreak spreads throughout an area where the disease wasn’t previously known
  • when people who weren’t previously susceptible to an infectious agent suddenly start getting sick from it

Smallpox, cholera, yellow fever, typhoid, measles, and polio are some of the worst epidemics in American history. Today, HIV and drug-resistant tuberculosis are considered epidemics.

Scholars date the use of the term epidemic as far back as Homer’s “Odyssey,” in which the poet used the term in a way that’s similar to the way we now use endemic.

The first recorded instance of the word epidemic being used to refer to a widespread disease is around the year 430 B.C., when Hippocrates included it in a medical treatise.

Today, the word epidemic is used in casual conversations to refer to almost anything negative that has spread throughout a culture or a region. For example, laziness, gun violence, and opioid use have all been called epidemics in popular media.

What is an epidemiologist?

Epidemiologists are scientists and doctors who study the incidence, control, and prevention of infectious diseases.

In 2010, during the H1N1 pandemic, the WHO defined a pandemic as the worldwide spread of a new disease.

At the time, the WHO described six phases in the development of a pandemic:

  1. A virus circulates among animals not known to spread the disease to humans.
  2. The virus is detected in animals known to have spread viral diseases to humans.
  3. Animal-to-human contact causes a human to develop the disease.
  4. Human-to-human contact makes it clear a community outbreak could happen.
  5. Human-to-human spread of the virus happens in at least two countries in the same region.
  6. Community level outbreaks happen in a third country in another region. Phase six meant that a pandemic was occurring.

In 2017, the CDC released a Pandemic Intervals Framework roughly aligned to the WHO’s pandemic stages.

Although both the WHO’s phases and the CDC’s framework describe flu pandemics, looking at the stages is useful for understanding how public health officials respond to global health emergencies, including the current COVID-19 outbreak.

CDC’s Pandemic Intervals Framework includes the following steps:

  1. Investigation: Officials monitor cases of novel flu in humans or animals and assess the risk of the virus becoming a pandemic.
  2. Recognition: As it becomes clear that the virus could spread widely, public health officials focus on treating patients and controlling disease spread.
  3. Initiation: The virus spreads easily and for a prolonged period.
  4. Acceleration: As the spread speeds up, public health officials use community interventions such as physical distancing and school closures.
  5. Deceleration: The number of new cases consistently drops, and public health officials may reduce community interventions.
  6. Preparation: As the first wave subsides, health officials monitor viral activity and watch for secondary waves.

In February 2020, the WHO said it intended to stop using the term pandemic, and the organization has also stopped using the six-phase approach to classifying a pandemic.

Still, this year the Director-General readopted the term, citing public health concerns around the spread of the new coronavirus globally.

Other Key terms about diseases and populations

To help understand the distinctions between pandemic and epidemic, it’s important to define several related terms:

  • Endemic. An infectious disease is endemic when it’s always present in a particular region. In some economically underdeveloped countries where water treatment facilities are insufficient, cholera is endemic. In rural parts of Spain, recurring tick-borne fevers are endemic, and the WHO is working to eliminate malaria in 21 countries where it’s considered endemic.
  • Sporadic. When a disease breaks out in an irregular pattern, it’s considered sporadic. If sporadic outbreaks occur often enough in the same region, epidemiologists think the disease should be considered endemic to that area.
  • Outbreak. A spike in the number of cases of the same illness in an area — beyond what health officials expect to see — is an outbreak. Among epidemiologists, the terms outbreak and epidemic have sometimes been used almost interchangeably, though epidemics are often considered more widespread. An outbreak could be an unexpected uptick in cases where a disease is endemic, or it could be the appearance of a disease in a region where it hasn’t shown up before. An outbreak doesn’t have to be an infectious disease, though. Right now, the CDC is tracking an outbreak of U.S. vaping-related lung injuries.

A pandemic is an epidemic that has traveled internationally. In other words, a pandemic is simply a larger and more widespread epidemic.

Recent pandemics

While no illness in recent history has affected the entire planet quite like the current COVID-19 pandemic, there have been others this century. Here are a few:

2009: H1N1

Between 2009 and 2010, a novel flu virus labeled (H1N1)pdm09 emerged. Called “swine” flu by many people, the disease caused an estimated 12,469 deaths in the United States.

The virus still circulates today during flu season.

2003: SARS

Arguably the first pandemic of the 21st century, severe acute respiratory syndrome (SARS), a type of coronavirus, spread across four continents before it was contained.

Although there have been no new cases since 2004, SARS is still registered as an infectious agent with the potential to have devastating effects on public health.

1957: H2N2

From 1957–58, a disease sometimes referred to as the “Asian flu” killed approximately 116,000 people in the United States and 1.1 million worldwide.

1968: H3N2

In 1968, an influenza A virus with two genes from avian flu strains killed nearly 100,000 Americans and 1 million people around the world.

The H3N2 virus continues to mutate and circulate during flu seasons today.

1918: H1N1

The influenza pandemic that occurred in 1918 was the deadliest outbreak in the 20th century.

Roughly 1/3 of the world’s population contracted the virus, which killed 50 million people globally, including 675,000 in the United States alone.

Preparing for a pandemic
  • Establish a communication plan for members of your family.

If you have relatives in other states, in care facilities, or away at college, decide in advance how you’ll stay in touch during a time of crisis. Make sure you understand how your family members want to be cared for if they get sick, especially those who live with you or near you.

  • Stock up on essentials, including medications.

The Department of Homeland Security recommends that you have extra supplies of water, food, prescription medications, and over-the-counter remedies on hand. Check to be sure you also have other essential supplies like thermometers, disinfectants, and paper goods. States differ on whether pet stores are considered essential, so it’s a good idea to make sure you have a ready supply of the food they’re accustomed to eating, along with their medications.

  • Keep medical records handy.

Make sure you have access to electronic copies of your family’s medical records, including prescription information, so that doctors have as complete a picture of your health as possible. If someone in your family has designated you as the person to make healthcare decisions for them if they’re incapacitated, you’ll also need that legal document on hand.

The difference between an epidemic and a pandemic isn’t the severity of the disease, but the degree to which the disease has spread.

When a disease exists all the time in a specific region or among a particular population, it’s known as endemic.

When a disease spreads unexpectedly throughout a geographical region, it’s an epidemic. When a disease spreads to multiple countries and continents, it’s considered a pandemic.

In March 2020, the WHO declared COVID-19 a pandemic.