The World Health Organization has declared the Wuhan coronavirus a “global emergency”. (Getty)

Since it emerged at the end of last year, China’s deadly coronavirus has made headlines all over the world.

The previously unheard of strain has infected people in every region of mainland China, as well as crossing national borders into 22 other countries.

READ MORE: The chart that shows the rapid rise in China's coronavirus cases

With the virus reaching as far as the US, UK and Australia, the World Health Organization declared it a “global emergency” on Thursday.

Chinese authorities have confirmed at least 213 deaths and almost 10,000 cases.

However, writing in the prestigious, peer-reviewed health journal The Lancet, scientists from the University of Hong Kong have warned more than 75,000 people could have battled the infection in the city Wuhan alone.

Scientists said it is possible that epidemics could be already growing in multiple major Chinese cities. (Getty)

Wuhan, the capital of Hubei province, is the epicentre of the outbreak. It has 11million residents.

The virus is thought to have “jumped” from an animal to a human at a live seafood and animal market, which was promptly shut.

Chinese authorities confirmed last week the virus can be transmitted from person-to-person via sneezing, coughing or shaking contaminated hands.

To learn more, the Hong Kong scientists used “air, train and road data” to estimate the number of cases “exported” from Wuhan between December 31 and January 28.

They then estimated the number of infections in Wuhan from December 1 2019, before it was identified, to January 25.

READ MORE: With coronavirus confirmed in the UK, do masks help ward off infection? And how should they be worn?

Results suggest 75,815 people had been infected in Wuhan as of January 25.

Each patient is thought to have had the potential to infect 2.6 others, the scientists add.

More than 400 cases are said to have to been “exported” from Wuhan to Chongqing, some 422 miles away (680km).

The major cities of Beijing and Shanghai are thought to have “imported” 113 and 98 infections from Wuhan, respectively.

The scientists noted “chunyun” - a 40-day period of “extremely high air and train traffic across China because of the lunar new year Spring Festival” - began on January 10.

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If the transmission rate in Wuhan applies elsewhere, they predict “epidemics are already growing exponentially in multiple major cities of China with a lag time behind the Wuhan outbreak of about one–to-two weeks”.

The team fear “large cities overseas with close transport links to China” will become outbreak epicentres.

“This paper uses a mathematical model to assess how many cases may have already occurred in Wuhan and project forward to see when the epidemic there might peak and how many cases might be expected as well as assess the impact of control policies,” Professor John Edmund, from the London School of Hygiene & Tropical Medicine, said.

“The model suggests there have been tens of thousands of cases in Wuhan already, which is in line with estimates made by other groups.

“The forward projections are much more speculative, however, and are probably best viewed as indicative rather than hard forecasts.

“This is inevitable, as at such an early stage in an epidemic when there is so little information on the virus and how it spreads, it is very difficult to make accurate predictions.”

Wuhan and neighbouring cities have been on “lockdown” since January 23 or 24.

Many countries have also been screening arrivals for signs of the infection, which tends to trigger flu-like breathlessness and fever initially.

Critics argue, however, it can take days for symptoms to develop, allowing newly-infected passengers to be missed, Yahoo UK reported.

The findings come after scientists from Imperial College London estimated each patient in China infected between 1.5 and 3.5 people up to January 18.

Speaking at the time, Professor Mark Harris - from the University of Leeds - said: “One positive spin is if we are only aware of 5% of the total cases.

“The implication is 95% of cases have only resulted in either mild symptoms such that the infected people did not consider it serious enough to seek medical help or indeed the virus may be causing an inapparent infection.

“This would significantly reduce the apparent mortality and morbidity rates.”

A team from the University of Oxford predicted if a virus-ridden person were to arrive in the UK, there is more than a one-in-three chance they will pass it on.

Based on data collected between January 10 and 21, scientists from the Chinese University of Hong Kong estimated each patient could pass the virus onto between three and five people.

Research also revealed the new strain, 2019-nCoV, is infectious before a patient develops symptoms.

“That could certainly explain why the virus is spreading quicker than [severe acute respiratory syndrome] Sars”, Dr Robin Thompson, from the University of Oxford, told New Scientist.

“Sars took several months to cause a thousand cases.

“This has caused [almost] 3,000 cases in three weeks.”

READ MORE: Race is on to develop a vaccine against China's coronavirus

2019-nCoV is thought to be the seventh pathogen in the coronavirus class that can infect humans.

Others include the common cold, and deadly outbreaks like Sars and Middle East respiratory syndrome (Mers).

Sars killed 774 people, and infected more than 8,000, during its 2004 epidemic. No cases have been reported since.

Authorities were famously tight-lipped about the outbreak, not informing the WHO until February 11, when 305 cases - and five deaths - had occurred across six districts in the province Guangdong.

By March 21, suspected and probable cases reached 350 - including 10 deaths - in 13 countries.

On March 31, 1,622 people were infected, of which 58 died, according to Nature.

The first two Mers patients were identified in Saudi Arabia in September 2012, of which one died.

A third incidence was reported on November 4, according to the European Centre for Disease Prevention and Control.

In May, the infection reached France. Forty four cases were reported overall at that time, with 22 deaths.

Mers has infected 2,494 people and killed 858 “to date”. The virus had an outbreak in 2012, with a handful of cases still occurring.

BEIJING, CHINA - JANUARY 31, 2020: Local residents wearing medical masks in a street. An outbreak of pneumonia-like disease caused by a coronavirus was registered in Wuhan, a port city of 11 mln people, the administrative center of the Hubei province, in the end of December 2019. Now over 9.8 thousand people are infected across China. Roman Balandin/TASS (Photo by Roman Balandin\TASS via Getty Images)

How does the new coronavirus kill patients?

Fatal pneumonia comes about when a respiratory infection causes the alveoli (air sacs) in the lungs to become inflamed and filled with fluid or pus, according to the American Lung Association.

The lungs then struggle to draw in air, resulting in reduced oxygen in the bloodstream.

“Without treatment the end is inevitable,” said the charity Médecins Sans Frontières.

“Deaths occurs because of asphyxiation.”

Genetic analyses reveal 2019-nCoV is more closely related to Sars than any other coronavirus.

The BBC notes, however, the death toll of the new strain “remains far lower”.

The US Centers for Disease Control and Prevention (CDC) has warned there is no specific treatment for coronaviruses.

If the infection triggers pneumonia, doctors work to combat the complication.

Pneumonia is generally caused by bacteria, which tend to respond to antibiotics.

When a virus is to blame – like 2019-nCoV – it may be treated via “antiviral medication”, according to the American Lung Association.

Professor Peter Horby from the University of Oxford claims, however, there is “no effective anti-viral”, with treatment being “supportive”.