The World Health Organisation (WHO) on March 11 declared COVID-19 a pandemic, pointing to the over 118,000 cases of the coronavirus illness in over 110 countries and territories around the world and the sustained risk of further global spread.

“This is not just a public health crisis, it is a crisis that will touch every sector,” said Dr Tedros Adhanom Ghebreyesus, WHO director-general, at a media briefing. “So every sector and every individual must be involved in the fights.”

An epidemic refers to an uptick in the spread of a disease within a specific community. By contrast, the WHO defines a pandemic as global spread of a new disease, though the specific threshold for meeting that criteria is fuzzy. The term is most often applied to new influenza strains, and the CDC says it’s used when viruses “are able to infect people easily and spread from person to person in an efficient and sustained way” in multiple regions. The declaration refers to the spread of a disease, rather than the severity of the illness it causes.

In some ways, declaring a pandemic is more art than science. “Pandemics mean different things to different people,” National Institute of Allergy and Infectious Diseases Director Dr. Anthony Fauci said in February. “It really is borderline semantics, to be honest with you.”

During multiple prior press briefings, WHO officials maintained that COVID-19 had “pandemic potential,” but stopped short of declaring it one. The agency did, in January, call it a public health emergency of international concern, a slightly different label that refers to an “extraordinary event” that “constitute[s] a public health risk to other States through the international spread of disease.”

“Pandemic is not a word to use lightly or carelessly. It is a word that, if misused, can cause unreasonable fear, or unjustified acceptance that the fight is over, leading to unnecessary suffering and death,” said Dr. Tedros on March 11. “Describing the situation as a pandemic does not change WHO’s assessment of the threat posed by this coronavirus. It doesn’t change what WHO is doing, and it doesn’t change what countries should do.”

SARS-CoV-2, the virus that causes COVID-19, is thought to have first jumped from an animal host to humans in Wuhan, China. At least at first, most cases were seen within China and among people who had travelled there, as well as those travellers’ close contacts. While these cases were concerning, they did not suggest a pandemic, because there was not significant spread outside of China.

But as the total number of infections rose, so too did the number of cases that spread from person-to-person within communities around the world. Cases have now been confirmed on every continent except Antarctica, and secondary disease hotspots have emerged in places such as South Korea, Italy and Iran.

All of these factors helped inform the WHO’s pandemic declaration, which signals to the world that continued spread is likely, and that countries should prepare for the possibility of widespread community transmission. It may also inform travel policy, and prompt cities and countries to flesh out plans for quarantines and possible disruptions to public events if the need arises. It can also kickstart the expedited development of therapeutics and vaccines.

A pandemic declaration is fairly unusual. Severe Acute Respiratory Syndrome (SARS), a coronavirus related to COVID-19, and which infected about 8,000 people across 26 countries in 2003, did not reach pandemic status, for example.

The WHO last used the label during the 2009 H1N1 (or “swine flu”) outbreak but got pushback for that choice. Critics argued that the situation was not serious enough to warrant a pandemic declaration, and that giving it one caused unnecessary panic and precautions.