The coronavirus outbreak that originated in China has been considered a global health emergency for more than a month. Now, the World Health Organization says it’s a “pandemic.” Why?

Excellent question. Let’s review some epidemiology basics to find out.

Think back to the town where you grew up, and the infections that made the rounds. An “endemic” level of disease — or a community’s baseline prevalence of it — isn’t always zero. Consider chicken pox in the United States, or malaria in some African countries. Endemic disease certainly isn’t preferable, but it’s expected.

A sudden increase in the number of people with a particular disease in a limited area can signal the onset of an outbreak. So can the emergence of an unusual type of disease, such as a strain of the flu that’s never been seen before.


If an outbreak expands to a broader population, it becomes an epidemic.

The WHO’s decision to declare a “public health emergency of international concern” in early February wasn’t arbitrary. The global health agency considers three factors when weighing that designation:

1. Is the outbreak unusual or unexpected? With a never-before-seen virus like the coronavirus that causes COVID-19, the answer was yes.


2. Does the outbreak have implications for health beyond a national border? When the emergency was declared, there were dozens of reported cases on four continents, so the answer was another yes.

3. Does the outbreak require immediate international action? Since airport screenings and aid money were needed to attempt to contain the virus’ spread, the answer was once again yes.

But epidemics can be severe without ever rising to the level of a pandemic, so long as they predominantly affect one region — in this case, China.

The Centers for Disease Control and Prevention defines a pandemic as one that has spread to several countries or continents, usually affecting a large number of people. The severity of illnesses is not a consideration, nor is the number of deaths.


With more than 125,000 confirmed cases on six continents, the term “pandemic” clearly applies.

“Diseases are not terribly good at respecting borders,” said Alex Azar, secretary of the Department of Health and Human Services.

The new coronavirus has demonstrated that it can spread readily between humans. That makes it more similar to influenza — a classic pandemic agent — than to other coronaviruses.


Beyond the case count, a pandemic contrasts with an epidemic because it has global consequences.

That’s why Middle Eastern respiratory syndrome never became a pandemic. Since the coronavirus that causes MERS began circulating in humans in 2012, it has infected only about 2,500 people, mostly in and around the Arabian Peninsula.

On the other hand, severe acute respiratory syndrome earned pandemic status because it affected 26 countries. The coronavirus that causes SARS triggered ongoing chains of transmission not just in China, the epicenter, but also in Canada, Hong Kong, Taiwan, Singapore, and Vietnam.


The COVID-19 coronavirus could wind up following the path of the H1N1 swine flu, which caused a pandemic in 2009. That outbreak subsided in 2010, but the virus continues to circulate as one of the primary strains of the seasonal flu.

Sam Scheiner, a mathematical modeling expert for infectious diseases at the National Science Foundation. said that if the new coronavirus joins the list of seasonal respiratory viruses, it could ultimately lead to tens of thousands of deaths in the United States each year.