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Update: On Wednesday, March 11, the World Health Organization officially declared the coronavirus a global pandemic. Read about it here.

As of Monday, March 9, cases of the novel coronavirus have passed 110,000 worldwide, with outbreaks worsening in the US, Italy, and Iran (though easing in China and South Korea) — and more countries around the world announcing cases. In the US, there are now more than 600 confirmed cases, with numbers rising in Washington state, California, Boston, New York, and elsewhere as more and more states report cases within their borders.

The head of the World Health Organization said that the world is in “uncharted territory” with a dangerous and rapidly spreading disease that has caused serious outbreaks around the world. But there’s one word the World Health Organization has yet to use to describe the outbreak: “pandemic.”

“The threat of a pandemic has become very real,” WHO Director General Tedros Adhanom Ghebreyesus said in a briefing Monday — but still stopped just short of calling the situation an actual pandemic.

The WHO says that since there’s still uncertainty about the disease’s impact, most countries still have only a few cases, and containment is not yet hopeless, the word “pandemic” isn’t appropriate yet. But many observers have disagreed with the WHO’s decision, believing that what we’re seeing unfold now fits the definition of the word.

As the novel coronavirus continues its global spread, with cases in more than 100 countries and regions, and 20 countries reporting more than 100 cases, experts have drawn comparisons — some of them reassuring, some of them alarming — to past major disease outbreaks, public health emergencies, and pandemics, from SARS and MERS to the 1918-’19 Spanish influenza that was one of the biggest catastrophes of the past 200 years.

So: What makes a pandemic? Or an epidemic? And what’s the difference? This explainer will catch you up on the terminology and the history behind global disease scares.

What is a pandemic?

To define a pandemic, we need to first define a few other terms.

First, when a disease infects a lot of people in one area in a short period of time, that’s an outbreak. For example, the US has experienced a series of measles outbreaks over the past few years, with 2019 the worst year for measles since 1992.

The measles outbreaks were quite different from a global public health perspective than the coronavirus outbreak. Measles was introduced by travelers from other countries and spread in populations with low vaccination rates, instead of jumping from animals. Since there is a vaccine, and since some people have had exposure and immunity, the measles outbreaks — while a serious public health crisis — didn’t present a risk on the scale of the coronavirus outbreak.

The novel coronavirus crisis began with an outbreak of 41 identified cases in Wuhan, China, in December that began spreading rapidly in January.

That brings us to a term you might have seen thrown around: epidemic. The World Health Organization defines an epidemic as “the occurrence in a community or region of cases of an illness ... clearly in excess of normal expectancy.” The CDC defines it as “an increase, often sudden, in the number of cases of a disease above what is normally expected” in a region. You may have heard the term used to refer to the “opioid epidemic,” or other instances of a long-existing harm becoming rapidly more deadly or more widespread.

By that definition, the coronavirus outbreak has certainly reached “epidemic” status in a number of regions, most notably China, South Korea, Iran, and Italy. In a briefing last week, Ghebreyesus referred to it in those terms. “This epidemic is a threat for every country, rich and poor,” he said, calling on governments around the world to respond with the strongest measures available.

When it comes to defining a pandemic, things get a little more complicated.

According to A Dictionary of Epidemiology, the standard reference for epidemiologists, a pandemic is “an epidemic occurring worldwide, or over a very wide area, crossing international boundaries and usually affecting a large number of people.”

A “pandemic” disease sounds much scarier than a mere “outbreak.” But it’s important to remember that “pandemic” refers to how many parts of the world are dealing with an elevated rate of the disease — and, in theory, says nothing about how serious the disease is.

In 2009, the H1N1 influenza was deemed a pandemic. One in five people worldwide caught the disease. But it was not very deadly, with an overall mortality rate estimated at 0.02 percent, so the societal disruption was limited and the loss of lives, while tragic, didn’t overwhelm medical systems.

Past coronaviruses have been dangerous, but they have not gone pandemic around the globe. The 2002-’03 SARS outbreak — primarily in China and Hong Kong, though with cases around the world — had about a 10 percent mortality rate. But as the disease was swiftly contained, it was never a pandemic, and no cases have been reported anywhere in the world since 2004.

Similarly, the even deadlier MERS coronavirus — with an approximately 35 percent mortality rate — was not very contagious and circulated only slowly. MERS originated in Saudi Arabia, and nearly all transmission has been there, though a traveler from the Middle East spurred a 2015 outbreak in South Korea. It has not been entirely eliminated, but it has never been a pandemic.

Sometimes, though, diseases have both a high mortality rate and enough transmissibility to become a pandemic. That’s the worst-case scenario for public health. The best example is the 1918-’19 influenza pandemic, which — aided by the troop movements of World War I — travelled all around the world and struck an estimated 500 million people.

At the time, that represented 25 percent of the population of the world. There are varying estimates for its lethality, but it was likely lower than that of SARS or MERS and may even have been as low as a few percentage points. That’s still high enough to have had a devastating effect on society. The death toll is estimated to have been 40 million to 50 million, though some estimates are higher.

When public health experts talk about their fear of another global pandemic, they have in mind the possibility of something like the 1918-’19 influenza. That said, it’s worth noting that even a “mild” pandemic like H1N1 in 2009 is estimated to have caused up to 575,000 deaths, because when a disease infects the whole world the death toll will be very high even if the disease is fairly mild.

So is the new coronavirus a pandemic?

By that standard, the novel coronavirus outbreak seems at first glance like it really ought to be considered a pandemic. It is an epidemic in China, Italy, South Korea, and Iran, and it is occurring in more than 80 countries, with some devastating effects. More than 100,000 people have been affected so far (and millions more have been affected by travel restrictions and quarantines).

Indeed, many global health experts have called it a pandemic. The outbreak “has become a global pandemic,” German health minister Jens Spahn said Wednesday.

“Personally, I think we’re doing everyone a disservice by continuing this debate [over whether it’s time to call the situation a pandemic],” Lauren Sauer, director of operations for the Johns Hopkins Office of Critical Event Preparedness, told the Washington Post. “It is creating more panic than just declaring it and moving on.”

But the World Health Organization has still not used the word “pandemic” for the outbreak. Ghebreyesus, the WHO’s director general, has said the decision will be based on “an ongoing assessment” of how widely the virus has spread and how badly it is affecting people. He cited the fact that most cases have been in China and that case counts have been very limited in most countries.

“Although a few countries are reporting large numbers of cases, 115 countries have not reported any cases. Twenty-one countries have reported only one case. And five countries that had reported cases have not reported new cases in the past 14 days,” Ghebreyesus argued Thursday, although those numbers have since changed.

So according to the WHO, the coronavirus spread isn’t a pandemic yet — though it may become one if things get worse.

The WHO has held off before on using the word “pandemic,” even once it was applicable. In 2009, with the spread of H1N1, the WHO was reluctant to declare the situation a global pandemic, despite the fact that the disease ended up infecting between 10 percent and 20 percent of the world in both hemispheres, which clearly qualifies it as an “epidemic occurring worldwide, or over a very wide area.”

Experts observed at the time that the WHO’s guidance on categorizing epidemics seemingly had political considerations. The “complicated pandemic definitions” that the WHO and the CDC had deployed seemed crafted “in a deliberate attempt to garner political attention and financial support for pandemic preparedness,” researcher Heath Kelly argued in the Bulletin of the World Health Organization.

In other words, rather than using the classic definition of a pandemic, the WHO found itself leaning on its a definition that mixed severity and spread. The WHO was aware that people associated the word with disease severity, and wanted to be using language that reflected that so the WHO would be taken seriously in requests for pandemic preparedness.

It seems likely that there’s something similar is going on right now. The word “pandemic” has profound implications for how we think about a disaster, and the World Health Organization is wary of them.

“Using the word pandemic carelessly has no tangible benefit, but it does have significant risk in terms of amplifying unnecessary and unjustified fear and stigma, and paralyzing systems,” Ghebreyesus argued last week.

Other organizations have mostly followed the WHO’s lead. In some cases, the situation has been further complicated by local political constraints on how to talk about the virus. Earlier this week, Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said the virus would reach “likely pandemic proportions,” reportedly angering the White House.

Why it matters that something is called a pandemic

So if the distinction between a pandemic and an epidemic isn’t how severe they are or how scared we should be, what is the distinction? Why bother categorizing diseases by how many countries are affected?

One answer is that pandemics must be battled, at the international level, significantly differently than epidemics.

When one region of the world experiences an epidemic, the rest of the world is on the sidelines. They may shut their borders to the affected region, or send help, or both. They may start making preparations in case the disease affects their own country. When coronavirus broke out in Wuhan, many of China’s neighbors sent masks. In a now-retracted Lancet paper, health workers in Wuhan asked if they’d consider sending volunteer nurses and doctors, too.

When there’s a global pandemic, there largely are no sidelines. It no longer makes sense for countries to shut their borders against the affected countries, as the disease is present everywhere. Public health officials might recommend shifting to social distancing measures — which reduce spread within a country — instead of screening efforts that try to keep the disease out of the country at all. Countries still must share medical knowledge and expertise with one another, but directing supplies to affected areas is more complicated when affected areas are everywhere. A world fighting a pandemic is not focused on containment to a distant elsewhere but on harm-reduction at home.

By that definition of a pandemic, we’re at a tipping point. Some experts say it’s already too late for containment, but not all experts agree. The WHO does not, arguing in Monday’s press conference that “the threat of a pandemic has become very real. But it would be the first pandemic in history that could be controlled. The bottom line is: we are not at the mercy of this virus.”

This is why we’re seeing disagreement on whether we’ve now reached pandemic stage with the novel coronavirus.

The first two months of the coronavirus’s spread have been full of surprises. It makes sense the World Health Organization wants to choose its words carefully — and that it’s still holding out hope that containment can be achieved.

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