As the number of people diagnosed with the new coronavirus tops 8,000, the World Health Organization (WHO) took the rare step to declare it a global emergency on Thursday.

The Geneva-based UN agency, has only made the declaration five other times.

The status of global emergency was established as a way to signal a serious worldwide public health threat in 2005, under a binding international legal agreement involving 196 countries, including all of its member states.

The designation did not exist at the time of SARS, but was developed after both that outbreak and bird flu made the need for better international co-operation clear.

The WHO held off on calling an emergency last week because of the limited spread of the virus outside of China. There have been cases reported in more than a dozen countries of the disease, called 2019-nCoV virus, and 170 deaths.

“So far, we have not seen any deaths outside China, for which we must all be grateful. Although these numbers are still relatively small compared to the number of cases in China, we must all act together now to limit further spread,” said WHO director general, Dr. Tedros Adhanom Ghebreyesus, in a statement.

The “vast majority” of people outside of China who have contracted the virus have a history of travelling to Wuhan or have had contact with someone who has recently travelled there. But, said Ghebreyesus, “we don’t know what sort of damage this 2019-nCoV virus could do if it were to spread in a country with a weaker health system.

“We must act now to help countries prepare for that possibility,” he said.

The WHO defines a global emergency as an “extraordinary event” that constitutes a risk to other countries and requires a co-ordinated international response. (It can also be called for other types of public health risks across borders, such as nuclear accidents.)

The declaration triggers “non-binding, but practically and politically significant measures that can address travel, trade, quarantine, screening, treatment. WHO can also set global standards of practice,” the agency tweeted after the meeting of its emergency committee Thursday afternoon.

The designation increases the level of urgency, and is designed to rally the international community to form a co-ordinated response, and apply more money and resources to it.

Here’s a look at when the WHO has decalerd a global emergency in the past:

Swine Flu 2009

The first time the WHO invoked a global emergency was in 2009 for the Swine Flu, also known as H1N1, which started in Mexico.

It took the agency only a month after labs isolated the virus to declare the emergency, in April of that year.

The WHO was later accused of overreacting and creating unnecessary alarm with its declaration.

Polio 2014

Polio was discovered in 1894, and there’s been a safe and effective vaccine for decades.

But, in May 2014, the WHO announced that the spread of the disease represented an “extraordinary event,” after cases in a few countries, including Pakistan and Afghanistan.

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The virus can cause permanent paralysis and death.

“If unchecked, this situation could result in failure to eradicate globally one of the world’s most serious vaccine-preventable diseases,” the emergency committee wrote at the time.

Ebola 2014

The agency was criticized for not acting fast enough to address the spread of the deadly virus, which eventually killed more than 11,000 people in West Africa

The 2014 epidemic started in Guinea and quickly spread to Liberia and Sierra Leone.

The virus was first detected in March of that year, but an emergency was not declared until August, by which time almost 1,000 people had died.

Zika 2016

The Zika virus was first detected in Uganda in 1947, and, as is malaria, is spread by mosquito bites.

It usually doesn’t cause serious illness in adults, but can have devastating consequences for pregnant women.

Brazil declared a national emergency in November 2015, over the virus, after an alarming spike in microcephaly cases, a condition where babies suffer from abnormally small heads and incomplete brain development.

WHO faced some criticism for not declaring an emergency until February of the following year.

Kivu Ebola 2019

The WHO last made an emergency declaration in July 2019 in response to an outbreak of Ebola in the Democratic Republic of Congo that continues.

According to Doctors Without Borders, the outbreak is centred around the northeast region of the country, and is the second-biggest Ebola epidemic ever recorded, after the West Africa outbreak of 2014-2016.

As of this month, there are 3,395 cases and 2,235 deaths, although the number of cases stabilized somewhat at the end of the year.

—with files from Star wire services