By now, everyone has heard the term “pandemic.’’ We’ve been hearing the term in news reports since COVID-19 was declared a pandemic by the World Health Organization on March 11, 2020 and the virus had spread around the world. What does pandemic mean, exactly? What is the difference between a pandemic and an epidemic? And what does “endemic” mean?

The meaning of an “epidemic’’ and a “pandemic’’

The Centers for Disease Control defines an epidemic as an outbreak where there is a sudden increase in cases. So, for example, when COVID-19 began spreading quickly in Wuhan, China, that was an epidemic.

An epidemic becomes a pandemic when the disease spreads across several countries and affects a large number of people. The prefix “pan’’ comes from a Greek word that means “all’’ or “of everything.’’ COVID-19 became a pandemic once it spread around the world to both hemispheres.

What does “endemic” mean?

The CDC defines “endemic” as a constant presence of a health condition within a certain area – it’s a condition that never goes away. For example, influenza is “endemic." Each year, in the United States, we experience “flu season.’’ The good news about influenza is that the flu vaccine is widely available, and while flu can be a serious illness for some people, we have drastically reduced the number of serious illnesses and deaths by making the vaccine a routine offering each fall and winter.

At some point, scientists believe COVID-19 will be endemic in the United States and perhaps around the world. By the time it becomes endemic, we hope COVID-19 will be much better controlled because of the increasing availability of vaccines.

Can an infectious organism that is endemic be eliminated?

Yes, it is possible. For example, in 2000, measles was declared eliminated from the United States. The World Health Organization (WHO) defines elimination of measles as “the absence of endemic measles virus transmission in a defined geographical area (e.g., region or country) for at least 12 months in the presence of a surveillance system that has been verified to be performing well.”

Declaring measles “eliminated’’ in the United States means the disease is no longer constantly present in this country. It’s important to note, however, that travelers can bring measles into the country, and it can sometimes spread and cause outbreaks among people who are not vaccinated. Thus, vaccination against measles remains critical to prevent outbreaks from occurring. If a measles outbreak continues for a year or more, the United States could lose its measles elimination status.

The risk for most people in the United States of getting measles is low, however, because most people are vaccinated against it.

What lessons can we learn?

Until COVID-19 becomes controlled, like influenza, and we reach herd immunity, we need to continue following all the best practices to control spreading the disease. Keep those masks on when you’re out in public. Make sure your mask fits snugly and covers your mouth and nose. Continue to practice social distancing and frequent, thorough handwashing.

And when the vaccine becomes available to you, remember that widespread vaccination of our community is the best way to keep everyone safe and to end the COVID-19 pandemic.