What Is the Difference Between an Endemic, Epidemic, and Pandemic?

The COVID-19 pandemic has brought up some new words about viruses you might not have heard before, and though they might sound similar, there are key differences. Here’s what you should know.

by Beth W. Orenstein Health Writer

You flip on the TV—there’s COVID-19 talk. You scroll through headlines on your phone—COVID-19. Your news app notifications are filled with, you guessed it, the latest on COVID-19.

Reports related to COVID-19 have dominated the news almost every day for more than a year at this point. During all this coverage, you’ve probably heard it referred to as an epidemic, pandemic, and endemic—but despite the way these terms get tossed around, they’re not the same. In fact, there are some key differences among them. Consider this your cheat sheet on the vocabulary of viruses.

How Is a Disease Outbreak Related to an Epidemic?

Before we can start talking about how a disease becomes an epidemic or pandemic, we need to understand how it gets a foothold in the general population. It all starts with an outbreak, says Tina Q. Tan, M.D., a pediatric infectious diseases physician and medical director of the International Patient and Destination services program at Lurie Children’s Hospital in Chicago and pediatrics professor at Northwestern University Feinberg School of Medicine.

“An outbreak is when you have a dramatic, sudden increase of a disease in a certain time period and geographical area,” Dr. Tan says. “It’s a significant increase above what might be considered baseline.” Take measles, for example. “Normally, measles does not circulate in the population here in the U.S. because we have high enough vaccination rates to prevent circulation,” she says. “If all of a sudden we are seeing multiple cases of measles in an area, that would be considered an outbreak.”

An outbreak can last for a short period of time (several days) to longer ones (a few weeks to up to many years), according to the Association for Professionals in Infection Control and Epidemiology (APIC). The type of disease that causes an outbreak is often a virus. That’s because viruses tend to spread more easily, especially if they’re transmitted via a respiratory route (air). However, outbreaks can be bacterial as well. A good example of a bacterial outbreak is salmonella.

When Does an Outbreak Become an Epidemic?

When does an outbreak become an epidemic? It’s all about size. “An epidemic is when you see more cases of disease than expected in a given area or among a specific group of people over a particular time period,” says Richard A. Martinello, M.D., an infectious disease expert at Yale Medicine in New Haven, CT. “Scientists tend to use ‘epidemic’ for a larger geographic area and ‘outbreak’ for a more limited geographic area.”

Interestingly, there are no hard-and-fast rules about how many people need to be infected for an event to be called an epidemic. Take the flu, for example. “Typically we see an outbreak of the influenza virus across the country every winter, and it is declared an epidemic,” Dr. Martinello says. Yet, over the last decade, yearly cases have varied between 9.3 million and 45 million in the United States, according to the Centers for Disease Control and Prevention (CDC). (This year, because of masks, lockdowns, and social distancing to prevent the spread of the coronavirus, the flu is nowhere near as widespread as in other years, Dr. Tan says.)

The Difference Between Epidemic and Pandemic

Simply put, a pandemic is a global epidemic, Dr. Martinello says. “‘Pan’ refers to the whole globe,” he explains. Pandemics not only tend to infect a much larger number of people than epidemics, but they also often cause social disruption and economic and other hardships because of their widespread nature, according to the APIC.

Also, pandemics are generally caused by a novel pathogen (see: SARS-CoV-2), Dr. Martinello says, meaning one that hasn’t been seen before and for which people have no immunity—that’s why it spreads so rapidly among the population. Before the COVID-19 pandemic, the last time the World Health Organization (WHO) declared a global pandemic was with a novel H1N1 virus in 2009. This H1N1 virus, like COVID-19, was a novel influenza virus. Dr. Tan says, “H1N1 was declared a pandemic because we were seeing this flu strain in very large numbers over multiple countries around the world.” It was first detected in the United States, and then spread quickly across the country and around the world.

An H1N1 virus also was the cause of the 1918 influenza pandemic that infected 500 million people around the world and left some 50 million dead, according to the CDC. It was the deadliest pandemic of the 20th century. COVID-19 could turn out to be the deadliest of the 21st century, but that won’t be known until the end of 2100.

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Are Pandemics on the Rise?

Thanks to COVID-19 and a 24/7 news cycle, it can feel like pandemics are suddenly everywhere. It’s true that with international travel so common these days, organisms can spread globally quite rapidly. However, international travel is not a requirement for a pandemic and doesn’t necessarily result in a pandemic. “I would not say that pandemics are more common,” Dr. Tan says. “I think it’s just that they are better recognized now.”

Similarly, pandemics do not seek out urban versus rural areas—even though it may appear that way. “Pandemics do not cluster more in urban areas, but they are more recognized because of an increased number of people to which the disease can spread and increased resources for testing,” says Dr. Tan. “Persons in all areas, whether urban or rural, can be and are affected.”

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What Does Endemic Mean?

If epidemic is national and pandemic is global, endemic must be… what? It can get confusing, Dr. Martinello concedes. Epidemiologists use the term endemic to refer to a disease that is always present in a certain population or area, he says, rather than something new.

The best example of an endemic is the common cold. “Every year, especially during winter, people get colds,” Dr. Tan says. “Viruses that cause colds are in the community. They don’t really go away. When the situation is ripe, they basically cause an infection, which causes a runny nose, cough, fatigue, and fever.”

If you’re thinking, “Hey, that sounds an awful lot like an epidemic though…,” well, you would not be wrong. Sometimes, endemic and epidemic can be used together, Dr. Martinello says. “For example, influenza may be endemic to a community, meaning it’s in that community and spreading within that community. You also could correctly say that the community has a flu epidemic,” he says. Could you use either endemic or epidemic in this case? You bet.

Bottom Line on Endemics, Epidemics, and Pandemics

The first case of the severe acute respiratory syndrome now known as SARS-CoV-2 was reported to WHO in December 2019. A month later, WHO declared the COVID-19 outbreak a global health emergency, but it wasn’t until March 11, 2020, that WHO declared it a global pandemic. “WHO—at least publicly—was waiting to be sure that the novel coronavirus (never seen before in humans) was epidemic on five continents,” Dr. Martinello says.

Terminology also changes as new viruses evolve. HIV is an example of a past pandemic that is now referred to in different terms, Dr. Martinello says. First reported in the Democratic Republic of the Congo around 1920, when HIV crossed from chimpanzees to humans, by the mid-to-late ’70s, sporadic cases were reported around the globe. By 1980, it had spread to the continents of North America, South America, Europe, Africa, and Australia, and some 100,000 to 300,000 people were infected. Because it was global, it was declared a pandemic at the time. “But four decades later, we’ve learned a great deal about prevention and treatment, and while it’s still endemic in some areas and an epidemic in others, it’s rarely referred to as a pandemic,” Dr. Martinello says.

Usually, the way that a pandemic ends is that “it kind of weaves its way into our day-to-day,” Dr. Martinello says. “While COVID will never disappear entirely, as more and more people are vaccinated and more and more people develop immunity to it, people won’t be getting as sick from it anymore, and fewer will die from it. As the years go by, it’s no longer going to be ‘novel’ to us, and the severity of the disease itself will hopefully be much, much less than we’ve seen over the past year-plus.”

The key is to be prepared. Says Dr. Tan, “Every single country in the world should have some infrastructure and protocols in place so we know how to handle the next novel pathogen that comes along and that could cause a pandemic.”

Understanding Epidemic Disease Occurrence: Centers for Disease Control and Prevention. (2012.) “Principles of Epidemiology in Public Health Practice, Third Edition.” https://www.cdc.gov/csels/dsepd/ss1978/lesson1/section11.html

Beth W. Orenstein
Meet Our Writer
Beth W. Orenstein

Beth W. Orenstein is a freelance medical writer based in Northampton, Pennsylvania. A magna cum laude graduate of Tufts University, Orenstein has written for HealthDay, EverydayHealth.com, and the National Psoriasis Foundation, and is a regular contributor to American Legion Magazine's Living Well and Radiology Today.