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It started late last year in a seafood market in the sprawling Chinese city of Wuhan. A mysterious virus previously unknown to science has since left six people dead and hundreds more sick. Cases have spread to Japan, South Korea and Thailand as Chinese authorities confirm that the illness, which causes pneumonia-like symptoms, can be passed from person to person.

China's President Xi Jinping has vowed to do everything possible to contain the outbreak. But, with hundreds of millions of people preparing to travel for the nation's biggest annual holiday, Lunar New Year, countries are scrambling to screen their borders, including Australia.

By January 21, the US confirmed the virus had spread to America, and the World Health Organisation was convening an emergency meeting to consider how big a threat the outbreak poses.

So what is a coronavirus, how does it spread and is this the world's next SARS?

Travellers wear face masks in Beijing. China as reported a sharp rise in the number of people infected with coronavirus.Credit:AP

Coronaviruses are a broad family of viruses that cause respiratory illness. They are mostly found in animals only six have previously been identified in humans including SARS-CoV, which led to the deadly SARS (Severe Acute Respiratory Syndrome)outbreak in 2003.

This new virus brings that tally to seven. Known as 2019-nCov (for novel coronavirus), the strain emerged in December, and has been linked to a seafood market in the capital of China's central Hubei province, Wuhan. The market has since been closed.

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Symptoms of the virus range from coughing, fever and a sore throat to more serious complications like pneumonia. At least four people diagnosed with the illness have died and many remain in a serious condition.

Scientists have unlocked the genetic code of the virus in "record time" - with cooperation from China - according to Robert Booy from the National Centre for Immunisation Research and Surveillance at the University of Sydney. But it is still unclear how easily and rapidly the illness will spread.

Experts are cautious when assessing its danger, noting so far other known coronaviruses have a higher mortality rate.

Professor Booy says about 10 per cent of people with SARS 800 died and MERS (Middle East Respiratory Syndrome) has proven even more deadly, with about 35 per cent of cases resulting in death.

On January 22, the Morrison government said there had been no diagnosed cases in Australia so far, despite a number of false alarms. That includes a man in Queensland who was temporarily held in qurantine for testing after he became ill following recent travel to Wuhan.

None of the known cases so far world-wide have involved children and at least one of the initial three people who had died also had an underlying disease, the government said in a statement.

The Coronavirus seen under an electron miscroscope.Credit:AFP

Coronaviruses are commonly carried by bats and then passed onto humans through mammals sold at live animal markets, says Professor Booy. Animals appear the most likely cause of this outbreak too.

Previously, Chinese authorities had claimed it could only be passed from animal to humans. But it's been reported at least two of those infected never set foot in Wuhan during the outbreak. Zhong Nanshan, a top Chinese expert investigating the virus, told state media they had caught it from family members and medical workers had also been infected. He warned it was now certainly a "human-to-human transmission phenomenon".

While Wuhan is believed to be the epicentre of the virus, it has spread to other Chinese cities including the capital, Beijing. Four cases have been reported outside China so far, in Japan, Thailand and South Korea. On January 21, Queensland authorities revealed they had quarantined a man for testing in Brisbane, after his recent travel to Wuhan.

The next three weeks or so will be critical to the world's understanding of the virus, says infectious disease specialist at the Australian National University Sanjaya Senanayake. But so far he says the outbreak has been milder than when SARS exploded onto the scene. Still, it has hit at the worst possible time for China amid Lunar New Year celebrations.

"So many people will be travelling, including here to Australia," Associate Professor Senanayake says. "I don't think we need to panic but I'm more worried now we're learning people outside the infamous seafood market have been infected."

Some experts predict the severity of the outbreak will depend on how many people, on average, a person with the virus can infect. Senanayake says he expects it will play out in a similar fashion to SARS people will catch it the way they catch a cold, from contact with infected people, animals or contaminated surfaces.

"With SARS, there wasn't one group of people affected but how people reacted did depend on their health," he says. "Fortunately, in this case, more people are getting milder cases."

On January 22, the World Health Organisation will meet to decide just that weighing up whether or not to declare the outbreak a global health crisis, such as they did during the start of Ebola in the Congo and the emergence of Zika virus in the Americas in 2016.

Chinese authorities have so far maintained the outbreak is"still preventable and controllable" but the country will ramp up monitoring over the Lunar New Year. On state media, Zhong said there was no danger of a repeat of the deadly SARS epidemic so long as precautions were taken.

Zhong Nanshan, a respiratory expert, confirmed the virus could be passed between humans but said China's quarantine procedures could still contain its spread.Credit:AP

President Xi has instructed his government to promptly release information on the virus and deepen international co-operation. Many countries have begun screening for the virus at airports, including US and now Australia.

On social media, people are posting prevention advice such as wearing masks and washing hands. Some have cancelled their travel plans for Lunar New Year. Senanayake stresses masks are only effective at blocking airborne viruses for the first few hours and should be changed regularly.

The number of cases may well be much higher than official numbers - the Imperial College of London estimates it's likely there are at least 1700 cases, when those undiagnosed are included.

"It's winter in China, a lot of these [cases] will seem like coughs and colds," Senanayake says.

While communication was notoriously sparse from China during the SARS crisis, Senanayake says it has been markedly better this time around, and international screening and quarantine methods have also improved.

"Still we [the world] are vulnerable to a superbug or virus, and even this could mutate at any time."

The news the virus may have already reached Australia comes as no surprise to experts, who say its steady influx of Chinese travellers make it vulnerable to such an outbreak. But, fresh from a meeting with state and territory health officials, the country's chief medical officer Brendan Murphy said the risk to Australians still remains low and well-established mechanisms were in place if it increased. At Sydney airport, where three direct flights from Wuhan arrive each week, passengers will be met by biosecurity officers asking if they have any symptoms.

Prime Minister Scott Morrison said on Tuesday that the government had raised the travel advice for those travelling to Wuhan to level 2, urging them to excercise a high degree of caution.

At the Centre for InternationalSecurity Studies, Adam Kamradt-Scott says Australia's public health system is well-placed to care for anyone infected but only international cooperation would see the virus properly contained.

Given that there are direct flights between Sydney and Wuhan, which is currently the epicentre of the virus, there is a reasonable chance that we might see cases emerge in Australia."

With Rachel Clun and Dana McCauley

Sherryn Groch is the explainer reporter for The Age and The Sydney Morning Herald.

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What is coronavirus and how worried should we be? - Sydney Morning Herald