Update, March 11, 1pm ET: The World Health Organization has decided to designate the novel coronavirus outbreak as a pandemic, citing the rapid spread of Covid-19 to new countries and the exponential increase in cases over the last two weeks. In a press conference, WHO director Tedros Adhanom Ghebreyesus said that “pandemic is not a word to use lightly or carelessly,” and that “all countries can still change the course of this pandemic.” This story’s headline has been updated to reflect WHO’s statement.

In the middle of an outbreak of a new disease, words have great power.

So far in the spread of Covid-19, the World Health Organization has studiously avoided one word in particular: pandemic. At a press briefing on March 3, senior WHO official Michael Ryan cautioned against “the dangers of using the pandemic word.” The big worry is that it might prompt governments to needlessly or prematurely change their strategies in ways that could undermine their efforts at containment.

So what would it really mean to call Covid-19 a pandemic? Let’s break it down.

What is a pandemic?

WHO’s definition is “the worldwide spread of a new disease.” Although cases of Covid-19 have appeared “worldwide,” it’s not clear, at least outside of China, that there is sustained local transmission of the infection. In other words, most cases can still be traced to travel in hotspot areas.

On March 3, Anne Schuchat, principal deputy director of the US Centers for Disease Control and Prevention, told a Senate committee that “if sustained person-to-person spread in the community takes hold outside China, this will increase the likelihood that the WHO will deem it a global pandemic.”

What happens when a pandemic is declared?

Calling Covid-19 a pandemic wouldn’t necessarily trigger any specific policy change, in the way that WHO’s decision in late January to call it a “public health emergency of international concern” triggered the formation of special committees and freed up funding and other resources.

“It’s really just a matter of semantics,” said Juan Dumois, an infectious disease physician at Johns Hopkins All Children’s Hospital. “Calling it a pandemic will not change what the WHO’s recommended responses will be.”

Still, Dumois said, the appellation might influence the way countries confront the outbreak—and whether those changes would be an improvement is a matter of debate.

How could a single word change how countries approach the virus?

Public health officials have two basic approaches to controlling outbreaks: containment and mitigation. Using the word “pandemic” could encourage them to focus on the latter.

Containment happens at earlier stages of an outbreak, when it still might be possible to isolate at-risk populations and keep them from spreading the disease to the rest of us. The strategies for this include identifying the source, tracing contacts between individuals, and instituting quarantines and travel bans. That approach was largely effective in slowing the spread of SARS in 2002, and in the West Africa Ebola outbreak of 2014 to 2016.

But once isolating a disease is no longer practical, it may be time to pivot to mitigation. At that point, governments assume that infections can happen anywhere, and reduce the likelihood of spread by closing schools and recommending the postponement or cancelation of events where lots of people are in contact.

WHO typically reserves the “pandemic” label for new versions of the flu (the last time the term was used was for 2009’s H1N1 “swine flu” outbreak), which can spread between people so easily that containment is usually a lost cause. Pandemics, in other words, are typically thought of as being best fought with mitigation.

“Using the term would help in some peoples’ minds create that shift from containment to mitigation,” Dumois said. But there’s not yet a consensus among public health researchers on whether that shift is needed to address the novel coronavirus.

In a CNN column in late February, former CDC director Tom Frieden warned that a Covid-19 pandemic is “inevitable,” and recommended that countries begin to take mitigation steps. But internal WHO notes circulated on March 4 and shared with Quartz suggest the agency doesn’t currently see coronavirus as an either-or situation. “The world is in a containment phase and we’re seeing success,” the notes assert. But they go on to warn that “the binary choice between containment and mitigation is often not reflective of the ground reality,” and that a “blended approach may be the most effective at dealing with areas of varying rates of the virus.”

How could the pandemic moniker change other public responses to the virus?

WHO director Tedros Adhanom Ghebreyesus has warned that calling Covid-19 a pandemic could create unnecessary panic, or prompt runs on hospitals and healthcare supplies. WHO was criticized by some researchers in 2009 for calling swine flu a pandemic, which may have contributed to overburdened hospitals at the time.

On the flip side, calling Covid-19 a pandemic might strengthen the case for the World Bank to release funds from a $320 million pandemic bond it set up after the West Africa Ebola outbreak. The fund is designed to help developing countries respond to a crisis like this—although the bond won’t pay out until 12 weeks after an outbreak begins. For Covid-19, that won’t be until late March.

Annalisa Merelli contributed to this story.