Not all infectious disease terms are created equal, though often they’re mistakenly used interchangeably. The distinction between the words “pandemic,” “epidemic,” and “endemic” is regularly blurred, even by medical experts. This is because the definition of each term is fluid and changes as diseases become more or less prevalent over time.

While conversational use of these words might not require precise definitions, knowing the difference is important to help you better understand public health news and appropriate public health responses.

Let’s start with basic definitions:

• AN EPIDEMIC is a disease that affects a large number of people within a community, population, or region.
• A PANDEMIC is an epidemic that’s spread over multiple countries or continents.
• ENDEMIC is something that belongs to a particular people or country.
• AN OUTBREAK is a greater-than-anticipated increase in the number of endemic cases. It can also be a single case in a new area. If it’s not quickly controlled, an outbreak can become an epidemic.

A simple way to know the difference between an epidemic and a pandemic is to remember the “P” in pandemic, which means a pandemic has a passport. A pandemic is an epidemic that travels.

But what’s the difference between epidemic and endemic? An epidemic is actively spreading; new cases of the disease substantially exceed what is expected. More broadly, it’s used to describe any problem that’s out of control, such as “the opioid epidemic.” An epidemic is often localized to a region, but the number of those infected in that region is significantly higher than normal. For example, when COVID-19 was limited to Wuhan, China, it was an epidemic. The geographical spread turned it into a pandemic.

Endemics, on the other hand, are a constant presence in a specific location. Malaria is endemic to parts of Africa. Ice is endemic to Antarctica.

Going one step farther, an endemic can lead to an outbreak, and an outbreak can happen anywhere. Last summer’s dengue fever outbreak in Hawaii is as an example. Dengue fever is endemic to certain regions of Africa, Central and South America, and the Caribbean. Mosquitoes in these areas carry dengue fever and transmit it from person to person. But in 2019 there was an outbreak of dengue fever in Hawaii, where the disease is not endemic. It’s believed an infected person visited the Big Island and was bitten by mosquitoes there. The insects then transferred the disease to other individuals they bit, which created an outbreak.

You can see why it’s so easy to confuse these terms. They’re all related to one another and there’s a natural ebb and flow between them as treatments become available and measures for control are put in place — or as flare-ups occur and disease begins to spread.