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 more than 1860 people, infected close to 73,200 and led to the lockdown of millions of people in central China.

Cases of the flu-like illness have emerged in more than 20 countries,  from England to Egypt, surpassing those officially diagnosed during the deadly SARS outbreak of 2003 and sparking extraordinary rescue efforts by foreign governments, including Australia, to free citizens trapped in the centre of the crisis.

Australia has 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 hoping to persuade Beijing to relax China's notorious firewall so international students can start the university year overseas.

As the global race picks up to develop a vaccine, the World Health Organisation (WHO) has warned that the spread of cases beyond people who have travelled to China could be "the spark that becomes a bigger fire". So far 15 people have been diagnosed in Australia, across NSW, Victoria, Queensland and South Australia, though six have already recovered.

On February 11, the mystery illness was at last given a name: COVID-19. And it was given another title by the WHO: "public enemy number one".

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

What is COVID-19?

Coronaviruses are a broad family of viruses that cause respiratory illnesses.They are mostly found in animals – only six have previously been identified in humans including the common cold and the more unusual 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 initially as 2019-nCov (for novel coronavirus) and now COVID-19, 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 coughs and colds from the four most common coronaviruses affecting humans. But when a new strain jumps into the population from animals, often through the handling and slaughter of wildlife, it can be dangerous as there is little natural immunity to fight it off.

How can you catch COVID-19?

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". But a lot is still unknown. Like SARS, it can spread between humans but COVID-19 is also contagious during it's one-to-14-day incubation period – before a person shows symptoms.

Infectious disease specialist Sanjaya Senanayake says he expects the virus will spread in a similar fashion to SARS – people will catch it in the way they catch a cold, from close contact with infected people, animals or contaminated surfaces.

Health authorities say 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. While it is not thought to be highly transmissible on planes, people travelling in the same row as a confirmed case or up to two rows in front or behind are considered to have come into close contact and should self-isolate.

Initial calculations have suggested each patient with COVID-19 would likely infect about three others. WHO spokesman Tarik Jašarević now says the rate is 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.

The infection is not believed to survive long outside the body, meaning you can't catch it from a package from China and it is very unlikely to be transmitted via food. You also can't catch the new strain from pets, although it is still a good idea to wash your hands around animals.

The WHO says the key to minimising the epidemic is early containment. That means frequent washing of hands and self-isolating if sick or exposed to the virus directly. It can also include covering the nose and mouth with a mask 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 that masks are effective at blocking airborne droplets only for the first few hours and should be changed regularly.

What are the symptoms and how serious is COVID-19?

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

Anyone can fall ill following close contact with an infected person but, as with most respiratory illnesses, it seems those most at risk of deadly complications are the elderly and people with underlying conditions. Although younger adults have also died from the virus, very few children have so far been diagnosed or suffered serious cases – a phenomenon also observed during SARS.

The WHO estimates only about 20 per cent of those infected with COVID-19 so far have suffered serious complications and says the death rate (which has hovered at about 1 to 2 cent so far) is falling as more cases are discovered. A handful of people have died outside China, one each in Japan, Hong Kong, the Philippines, Taiwan and France.As of February 18, a total of 1860 people had died and about 12,500 people had been counted as having recovered from the infection worldwide.

Right now, treatment involves getting oxygen into the lungs, monitoring vital organs and treating any resulting complications. While parallels in severity have been drawn with the common flu, emerging coronaviruses such as COVID-19 and SARS can do more damage to the body. SARS punched "honeycomb-like" holes in patients' lungs and such lesions are appearing again in many COVID-19 cases. Antibiotics do not work on the virus as it is not caused by a bacteria.

Is COVID-19 as severe as the flu or SARS?

It took the new coronavirus 48 days to infect the first thousand people. By contrast, SARS took 130 days and MERS (Middle East Respiratory Syndrome) more than two years to infect a thousand people after it emerged in 2012.

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 has been circulating for eight years, and kills about a third of those who fall ill – out of about 2500 confirmed cases so far.

Experts have been cautious so far when assessing COVID-19's danger, noting cases appear milder overall than those two previous outbreaks, even as it begins to look increasingly like a pandemic — "the worldwide spread of disease".

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

Seasonal flu kills between 291,000 and 646,000 people worldwide each year, the US Centers for Disease and Prevention calculates. In the US, influenza has already killed 10,000 people 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%.

While new vaccines for influenza are developed each season, there is no inoculation against COVID-19 so far. But under an extraordinary push world-wide, led in part by Australia, clinical trials for the first vaccines could just be months away.

Where did COVID-19 come from?

Coronaviruses are commonly carried by animals such as bats and rodents and then passed on to humans through contact with blood, faeces and other bodily fluids. Wild animals packed together and then butchered in live markets throughout Asia can be incubators for viruses to evolve and jump species barriers – SARS was traced back to a colony of bats but was believed to have passed into humans via the Himalayan palm civet, an ancient species of mammal eaten as a delicacy in China.

Early work suggests COVID-19 is 96 per cent similar to a SARS-like virus discovered in bats in 2017 and a study by Chinese scientists which is yet to be peer reviewed claims to have found an even closer match – 99 per cent – to a strain found in the endangered pangolin, the most illegally trafficked animal in the world.

Hanging over the crisis is the shadow of SARS – which also spread from a live animal market in China and saw the authoritarian government accused of a cover-up as it hid patients from international view. As it steps up mass forced quarantines of the suspected infected, China has temporarily banned its wildlife trade. Conservation groups and experts are calling on China to make the ban permanent, warning the nation has not learnt the lesson of SARS.

How big is the risk to Australia?

Due to the "escalating threat of the virus", Australia has beefed up its border measures and banned travellers from China, although citizens, permanent residents and their dependents or spouses will be accepted in and urged to self-isolate. (The US, Italy, New Zealand and South Korea have introduced similar measures but the travel bans have been slammed as "truly mean" by China.)

While experts acknowledge that 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. All of the Australian patients so far had recently travelled from Wuhan, except for one person in NSW who had contact in China with a confirmed case.

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 have been held for a two week quarantine. Another 250-odd Australians were flown from Wuhan into the Northern Territory, where they are being isolated in a repurposed mining camp.

None of those aboard the two rescue flights have so far tested positive for the virus, authorities say. Those remaining in Wuhan are now pleading with the government to arrange another flight home. 

Outside mainland China, the most cases have been confirmed on a cruise ship that has now become a floating quarantine site off the coast of Japan. More than 450 people, at least a dozen Australians among them, have been diagnosed with COVID-19 on board the Diamond Princess. The cruiseliner was carrying almost 3700 people including more than 200 Australians when the illness emerged in early February, believed to have come from a passenger who boarded in Hong Kong.

Officials had been bringing those infected off the ship one by one but on February 17, the Australian government announced it would follow countries such as the US in pulling its citizens out of the quarantine early.Qantas will fly to Japan on Wednesday to pick up the passengers, who will then be isolated at a Darwin mining camp for a further two weeks.

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