Years ago, experts warned that a mysterious rampant illness could spark a global health epidemic.
The World Health Organisation (WHO) gave this illness the name "Disease X" in 2017 – not referring to a specific disease, but to any new unknown pathogen that could potentially cause a deadly pandemic.
The new coronavirus, also called Covid-19, is rapidly coming to fit that very profile.
'FIRST PANDEMIC THAT FITS DISEASE X'
The deadly virus, which started life in central China, has now infected almost 80,000 people around the world and killed over 2400.
The virus has spread well beyond China's tightly-restricted borders, infecting hundreds in South Korea and Japan, more than 150 in Italy, and more across the globe.
Unlike SARS, the coronavirus replicates at high concentrations in the nose and throat, and appears capable of spreading to those who show zero or mild symptoms.
Marion Koopmans, head of viroscience at Erasmus University Medical Centre in Rotterdam, and a member of the WHO's emergency committee, said this virus was becoming the first of its kind to fit the "Disease X" category.
"Whether it will be contained or not, this outbreak is rapidly becoming the first true pandemic challenge that fits the disease X category," she wrote in the journal Cell.
Around one-in-seven coronavirus patients develop pneumonia and breathing difficulties from the virus. A total of 5 per cent of patients who develop the coronavirus will have critical illness, including respiratory failure, septic shock and multi-organ failure.
The WHO, meanwhile, has warned the window of opportunity to stem the deadly outbreak is "narrowing".
WHO director general Tedros Adhanom Ghebreyesus said, "Although the total number of cases outside China remains relatively small, we are concerned about the number of cases with no clear epidemiological link, such as travel history to China or contact with a confirmed case."
In a paper for the American Medical Association, doctors said: "Unlike SARS, Covid-19 infection has a broader spectrum of severity ranging from asymptomatic to mildly symptomatic to severe illness that requires mechanical ventilation.
"Clinical progression of the illness appears similar to SARS: patients developed pneumonia around the end of the first week to the beginning of the second week of illness."
Before this coronavirus existed, Bill Gates had in 2019 warned that a pandemic that the world had not seen before would emerge.
Appearing in Netflix documentary The Next Pandemic, the billionaire warned the world was ill-prepared to deal with the implications of such a virus.
"If a disease comes along that we haven't seen before, typically it would take four or five years to come up with a vaccine against that disease. And new technologies might shorten those times," he said. "When a pandemic comes along of any size, we always look back and wish we invested more."
CHINA'S 'LARGEST EVER PUBLIC HEALTH EMERGENCY'
Chinese leader Xi Jinping said the new coronavirus epidemic is the country's largest-ever public health emergency, but other nations were also increasingly under pressure from the deadly outbreak's relentless global march.
Italy and Iran began introducing the sort of containment measures previously seen only in China, which has put tens of millions of people under lockdown in Hubei province, the outbreak's epicentre.
Italy reported a third death while cases spiked and the country's Venice carnival closed early.
Iran's confirmed death toll rose to eight, prompting travel bans from neighbouring countries.
The virus has so far killed more than 2400 people, with about 80,000 infected globally, though China remains by far the worst hit.
President Xi Jinping said the epidemic was the "largest public health emergency" since the founding of the People's Republic in 1949.
"This is a crisis for us and it is a big test," he said during remarks carried by state television.
In a rare admission, at a meeting to co-ordinate the fight against the virus, Mr Xi added that China must learn from "obvious shortcomings exposed" during its response.
The World Health Organisation (WHO) has praised Beijing for its handling of the epidemic, but China has been criticised at home for silencing early warnings from a whistleblower doctor who later died from the virus.
South Korea said it was raising its alert to the highest level, after the number of infections nearly tripled over the weekend to 602.
The country now has the most infections outside of China, apart from the Diamond Princess cruise ship docked in Japan.
South Korea reported three deaths on Sunday, taking the countrywide fatality toll to five. The Yonhap news agency later reported a sixth death.
Around half of South Korea's cases have been linked to the Shincheonji Church of Jesus sect in the southern city of Daegu, where thousands of members have been quarantined or asked to stay at home.