“Epidemic” vs. “Pandemic”: What Do These Terms Mean?

If you recall the SARS outbreak in the early 2000s or are taking preparations against COVID-19 right now, then you’ve definitely heard the words epidemic and pandemic. With every biological outbreak, we encounter these words being used more and more frequently—and often, inaccurately.

Why is it so easy for people to confuse these words? Well, both words contain –demic and are used for disease outbreaks, but they’re not exactly the same. These similarities lead many people to use the two words interchangeably or incorrectly altogether. The key difference, however, is about scale. So, let’s explore the two.

What is an epidemic?

An epidemic disease is one “affecting many persons at the same time, and spreading from person to person in a locality where the disease is not permanently prevalent.” The World Health Organization (WHO) further specifies epidemic as occurring at the level of a region or community.

Epidemic is commonly used all on its own as a noun, meaning “a temporary prevalence of a disease.” For example: The city was able to stop the flu epidemic before it spread across the state.

Metaphorically, epidemic is “a rapid spread or increase in the occurrence of something,” usually with a negative or humorous connotation: An epidemic of gentrification was affecting low-income communities or The hipster look gave way to an epidemic of 1990s fashion.

The -demic part of epidemic (and pandemic) comes from the Greek dêmos, “people of a district.” This root also ultimately gives English the word democracy. More on the prefix epi– later.

What is a pandemic?

Compared to an epidemic disease, a pandemic disease is an epidemic that has spread over a large area, that is, it’s “prevalent throughout an entire country, continent, or the whole world.”

Pandemic is also used as a noun, meaning “a pandemic disease.” The WHO more specifically defines a pandemic as “a worldwide spread of a new disease.” On March 11, the WHO officially declared the COVID-19 outbreak a pandemic due to the global spread and severity of the disease.

While pandemic can be used for a disease that has spread across an entire country or other large landmass, the word is generally reserved for diseases that have spread across continents or the entire world. For instance: After documenting cases in all continents except Antarctica, scientists declared the disease a pandemic.

As an adjective, pandemic can also mean “general” and “universal,” also often with a negative connotation. However, pandemic appears to be most commonly used in the context of epidemiology, which is concerned with infectious diseases.

Pandemic also entered English, through Latin, in the 1600s. Like epidemic, pandemic ultimately derives from the Greek pándēmos, “common, public.” Also like epidemic, pandemic was originally used of diseases when in came into English.

How to use epidemic vs. pandemic

As we mentioned, it’s unsurprisingly easy to confuse these two words. For one, they both feature -demic, which can make it difficult to suss out which word should be used in which situation.

But, here’s a handy rule of thumb for using the prefixes of these two words: epi- and pan-. The prefix epi- is Greek and variously means “on, upon, near, at,” while pan-, also a Greek prefix, means “all.”

Knowing this, think of an epidemic as the start of something—whether a disease or a trend—spreading rapidly within a community or region, whereas a pandemic is what an epidemic becomes once it reaches a far wider swath of people, especially across continents or the entire world.

If something is spreading like wildfire, it’s an epidemic. If something has already spread like wildfire and is currently massive in its reach and impact, it’s a pandemic.

And for good measure … here’s another example of each in a sentence.

• The city had to close schools to contain a measles epidemic.
• Although it isn’t exactly known where the disease first originated, the 1918 Spanish flu pandemic is estimated to have affected one-third of people across the entire globe.