What is coronavirus and how worried should we be?

It appeared in a live animal market late last year in the sprawling Chinese city of Wuhan. Now a mysterious virus previously unknown to science has killed close to 600 people, infected more than 25,300 and led to the lockdown of 50 million in central China.

Cases of the flu-like illness have emerged in more than 20 countries, surpassing those officially diagnosed during the deadly SARS outbreak of 2003 and sparking extraordinary rescue efforts by foreign governments, including Australia, the US, France and Japan, to free citizens trapped in the epicentre of the crisis. Across China, cities have ground to a halt as the communist government moves to contain millions of people travelling for Lunar New Year.

On January 30, the World Health Organisation declared the virus a global health emergency – although experts note that, so far, it does not appear as deadly as similar outbreaks. Australia has since joined the US, Italy, New Zealand and South Korea in closing its borders to all travelling from mainland China but its own residents, and is even in talks with Beijing to relax China's notorious firewall so international students can start the university year overseas.

So far there are 14 confirmed cases in Australia, across NSW, Victoria, Queensland and South Australia, as the race picks up to develop a vaccine for the virus authorities say is already mutating.

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

What is a coronavirus?

Coronaviruses are a broad family of viruses that cause respiratory illnesses, so named for their distinctive crown or corona shape under a microscope. They are mostly found in animals – only six have previously been identified in humans including SARS-CoV, which led to the SARS (Severe Acute Respiratory Syndrome) outbreak almost 20 years ago.

This new virus brings that tally to seven. Known as 2019-nCov (for novel coronavirus), the strain emerged in December and has been traced back to a live animal market in the capital of China's central Hubei province, Wuhan. The market has since been closed and quarantined as the forensics of where the disease came from continue.

People often catch viruses causing colds and flu – including some known coronaviruses. But when a new strain jumps to humans, often through the handling and slaughter of wildlife, it can be dangerous as there is little natural immunity to fight it off.

What are the symptoms and how serious is it?

It appears to start with a fever, a cough or shortness of breath and can lead to more serious pneumonia-like complications.

Experts say medical authorities have unlocked the genetic code of the virus – which is about 75 per cent similar to the SARS strain – in "record time", thanks to cooperation from a Chinese government eager to avoid a repeat of the SARS crisis that saw it accused of a cover-up.

But a lot is still unknown and Chinese authorities have warned the new strain is already mutating as its spread picks up speed.Unlike SARS, it appears to be infectious during its one to 14 day incubation period – before a person shows symptoms.

Still experts have been cautious so far when assessing its danger noting cases appear milder overall than previous outbreaks, even as it begins to look increasingly like a pandemic  — an ongoing epidemic on two or more continents.

The WHO estimates only about 20 per cent of people infected with the new strain so far have suffered serious complications and says the death rate (which has hovered at about 2 to 3 per cent so far) is falling as more cases are discovered.

By February, the first death recorded outside of China was confirmed in the Philippines and another person has since died in Hong Kong. More than 1000 people have recovered from the infection worldwide, including two of the four people diagnosed in NSW.

When SARS finished its spread after nine months in 2003, only 8098 cases had been confirmed but close to 10 per cent of those were fatal. MERS (Middle East Respiratory Syndrome) has been circulating since 2012, and kills about a third of those who fall ill. Only about 2500 people are known to have caught it so far.

By contrast, the infamous 1918 "Spanish flu" killed about 2.5 per cent of its victims — but because it infected so many people and medical care was less advanced then, 20 to 50 million died. By contrast, the highly transmissible "swine flu" pandemic of 2009 killed about 285,000, fewer than seasonal flu normally does, and had a relatively low fatality rate, estimated at .02 per cent.

Is it as bad as the flu?

The US Centers for Disease and Prevention calculates that, world-wide, seasonal flu kills between 291,000 and 646,000 people each year. In the US, influenza has already killed 10,000 this season and in Australia, government figures for the last season ending October 2019 show 812 people died out of 298,120 reported cases - a fatality rate of about 0.27%.

Some experts have warned that while the totals might be higher for an established illness like the flu, new strains of coronavirus can cause more damage to the lungs.

Meanwhile, an Australian lab has become the first place outside China to successfully replicate the virus – a breakthrough that could now help the CSIRO begin testing vaccines within six weeks.

How is it spread?

Coronaviruses are commonly carried by bats and then passed on to humans through mammals sold at live animal markets. Chinese authorities initially claimed it could only be passed from animal to humans but, on January 20, they revealed that at least two people infected had never set foot in Wuhan during the outbreak. Zhong Nanshan, a top Chinese expert investigating the virus, told state media the people had caught the virus from family members. In one case, one patient was believed to have infected 14 medical workers. Zhong warned it was now certainly a "human-to-human transmission phenomenon".

Initial calculations put the infection rate at about 1 to 3 - each patient would likely infect about three ohers. On February 6, World Health Organisation spokesman Tarik Jašarević told The Sydney Morning Herald and The Age that rate was now estimated to be closer to four or more people. That makes it more infectious than the flu but much less infectious than measles, where one person is likely to infect up to 20 people.

While most cases are still on the Chinese mainland, the virus has been confirmed elsewhere in Asia including Thailand, Singapore and Japan as well as further afield in Europe, the Middle East and North America.

Hanging over the emergency is the shadow of SARS – which also spread from a live animal market in China and went on to kill nearly 800 people globally. That outbreak was eventually traced back to a colony of bats and was believed to have been passed onto humans via the Himalayan palm civet, an ancient species of mammal eaten as a delicacy in China.

Authorities suspect the new virus is also linked to the illegal wildlife trade. Wild animals packed together and then butchered in Chinese markets can be incubators for viruses to evolve and jump the species barrier to humans.

As it steps up containment efforts, China has banned the wildlife trade across the nation, including the shipping and sale of species. It has quarantined breeding sites and warned people against the consumption of wild animals. Conservation groups and experts are now calling on China to make the ban on wildlife trade permanent, warning the nation has not learnt the lesson of SARS.

How can you catch it?

infectious disease specialist Sanjaya Senanayake says he expects the virus will spread in a similar fashion to SARS – people will catch it the way they catch a cold, from close contact with infected people, animals or contaminated surfaces. NSW Health advises the virus can jump from person to person within at least 15 minutes of face-to-face conversation, or at least two hours in an enclosed space together. A spokeswoman said that when new confirmed cases of the virus emerged following air travel,  NSW Health contacted passengers who travelled up to two rows in front, two behind or in the same row as a person at the time they were infectious, as that was considered close contact.

Australian authorities are balancing caution with calls for calm, advising those who have recently travelled to central China or who have come into close contact with a confirmed case of the new virus to self-isolate for 14 days. Given symptoms mirror the common cold and flu, there have already been a number of false alarms. But those concerned should call ahead before visiting their GP and wear a face mask. Some jurisdiction such as NSW are even waiving medical fees.

The WHO says the key to minimising the epidemic is early containment. For the general public that means, frequent washing of hands and it can include covering the nose and mouth to prevent transmission of airborne droplets, though catching the virus in public spaces, such as from passing people in the street, is considered very unlikely. Senanayake notes masks are only effective at blocking airborne droplets for the first few hours and should be changed regularly.

"With SARS, there wasn't one group of people who caught it but how people reacted [to it] did depend on their health," he says. "I expect this will be similar."

Antibiotics do not work on the virus as it is not caused by a bacteria. As with most respiratory illnesses, it seems those most at risk are the elderly and people with underlying conditions.

So how is it being contained?

Large-scale quarantines are rare, even during deadly epidemics, and memories of the secrecy wielded by the Chinese government during the SARS crisis is now fuelling some distrust. For months, China parked SARS patients in hotels and drove them around in ambulances to conceal the true number of cases and avoid WHO experts.

This time around, China's leader President Xi Jinping has told officials to promptly pass on updates, put lives first and work with international agencies. "Anyone who deliberately delays and hides the reporting of [virus] cases out of his or her own self-interest will be nailed on the pillar of shame for eternity," a notice from Beijing's Central Political and Legal Affairs Commission reads.

On the ground, some people are speaking out about forced quarantines, even after testing negative for the virus. And on February 6 a Chinese doctor turned whistleblower who was silenced by police for trying to share early news of the mystery virus died from the disease himself.

Some lower-ranking Communist Party officials have started to admit they were slow to contain the virus. Since January 24, some 33 officials have been dismissed or scheduled for disciplinary review for allowing informal public gatherings, repeating misinformation via official channels, and failing to provide superiors with detailed updates.

Countries are still screening for symptoms at airports, as many airlines, including Qantas and British Airways, shutter flights from China. Italy has banned all air traffic from China and the Trump administration is barring entry to foreign nationals who have recently travelled to China.

Due to the "escalating threat of the virus" Australia has since beefed up its own border measures and followed suit with a ban on travellers from China, though citizens, permanent residents and their dependents or spouses will be accepted in and urged to self-isolate. New Zealand and South Korea have introduced similar measures but the travel bans have been slammed as "truly mean" by China.

On social media, people are posting prevention advice such as wearing masks and washing hands but misinformation and even xenophobia is also spreading fast. The WHO is working with Google to ensure that people get facts from the health organisation first when they search for information about the virus.

How big is the risk to Australia?

While experts acknowledge Australia is vulnerable to the outbreak –given its high influx of travellers from China – authorities have stressed it has good measures in place to deal with the virus.

"There is no reason for alarm in the general community," Victorian state health minister, Jenny Mikakos, said following the first revelation of an Australian case – a man in Melbourne with pneumonia.

Australia has airlifted hundreds of its citizens out of Wuhan to the Christmas Island detention centre where, having already run a 14-hour gauntlet of health checks in China, they will be held for a two week quarantine. All travellers on the first flight to island had been examined and cleared of infection, the government said, as it negotiated plans to board more Australians onto an upcoming New Zealand flight out of the Chinese city.

Back home, health authorities have beefed up their resources, stockpiling millions of masks as they work to trace people who shared flights with infected patients.  Fourteen people have been diagnosed with the virus and isolated, including an eight-year-old boy in Queensland.

With Dana McCauley, Rachel Clun, Eryk Bagshaw, Liam Mannix and The New York Times