What began as a mysterious cluster of pneumonia cases in the Chinese city of Wuhan has exploded into the world's next pandemic in little more than three months. Since December, a virus never before seen in humans has infected more than two million people across 180 countries and killed 165,000.

Cases in China have slowed dramatically under extraordinary lockdowns and testing regimes but new frontiers of the virus have erupted in Europe, the Middle East and the US, seeing more cities, from New York to Paris, grind to a halt. Now Australia is shutting large parts of daily life to slow the virus's spread here – including pubs, cinemas, gyms and many businesses – as states shutter their borders and call in police to enforce home quarantines for those at risk.

More than 6500 people have been diagnosed with the new illness, COVID-19, in Australia and about 70 people have died here (see graphic below). Health Minister Greg Hunt says the nation needs to adopt a "war-time spirit" and everyone is urged to do their bit by practising good hygiene and staying away from others (about 1.5 metres away) or at home where possible, especially if sick. Experts stress this "social distancing" is about slowing down the virus, and so avoiding a sudden surge of cases overwhelming hospital resources. It is not an apocalyptic call for the community to start hoarding supplies or face masks.

So what is a coronavirus, how does this one spread and how bad could it get?

This article was originally published on January 21, 2020 and has been updated to reflect new developments.

What is the coronavirus illness COVID-19?

Coronaviruses are a family of viruses causing respiratory illness that are mostly found in animals (so named for their crown shape, not the beer). Only six have previously been identified in humans including four mild strains which cause the common cold and the more unusual SARS-CoV, the virus behind the SARS (Severe Acute Respiratory Syndrome) outbreak almost 20 years ago. This illness, known as COVID-19, brings that tally to seven.

When a new strain of virus 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. COVID-19 first appeared in December, when people working and shopping at a live animal market in the capital of China's central Hubei province, Wuhan, started falling ill. Medical experts have unlocked the virus's genetic code – which is about 75 per cent similar to the SARS virus – in "record time". But a lot is still unknown.

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

It appears to start with a fever, a cough, fatigue or shortness of breath and can lead to pneumonia and more serious complications including organ failure. So far, with a death rate estimated at about 1 per cent, COVID-19 is considered 10 times deadlier than the seasonal flu but less dangerous than both SARS and the other rare coronavirus to emerge in humans, Middle East Respiratory Syndrome (MERS). As with those illnesses, it infects not just the nose and throat but the lungs as well. More than 600,000 people have already officially recovered from the illness worldwide, but like the mortality rate, this number is hard to track while the pandemic is still at full steam.

Anyone can catch COVID-19 but those most at risk of dying are the elderly and people with underlying conditions such as diabetes and heart disease. As of March 3, about 3.4 per cent of reported cases had been fatal, according to the WHO. Data of the first 55,000 or so cases in China reveals that about 20 per cent (one in five patients) suffered serious complications (meaning they needed medical intervention such as a ventilator to help them breathe).

Further analysis of cases in Europe and the US show people under 50 are also being hospitalised in concerning numbers but very few children have been diagnosed or suffered serious cases – a phenomenon also observed during SARS. Still, they are not immune and some have died from the illness.

Smokers are more likely to develop a nasty infection, as the virus breeds via cell receptors in the lungs known to increase with cigarette smoke. Experts say the higher the dose of virus you are hit with, the faster you will show symptoms and the more dangerous it can be for your immune system to fight off.

While the global race to develop a vaccine is picking up speed, right now there is no cure. Treatment involves getting oxygen into the lungs, keeping fluids up and dealing with any resulting complications. Antibiotics do not work on the virus as it is not caused by a bacteria but can be deployed if patients are hit by secondary bouts of bacterial pneumonia. More experimental drugs such as those developed to combat malaria, HIV and Ebola are also being trialled in some parts of the world.

Outside China, major clusters of the virus have broken out in Italy, where hospitals have been pushed to the brink as well as the US, Spain, Germany, Iran, France, the UK and South Korea. Many, including much of Europe, are now turning to the "China model" of forced quarantines and transport shutdowns to stop it. But experts note that China, as well as countries such as Singapore, Taiwan and South Korea, have also started to "flatten their curve" of infection growth through exhaustive testing, contact tracing of known cases and community take-up of "social distancing" measures such as working from home or taking schools online.

How do you catch COVID-19?

The virus needs us to spread – it can jump from one person to another through water droplets expelled from the nose and mouth, usually during coughing or sneezing (which can travel for close to a metre). Health authorities say you could be at risk if you touch a contaminated surface or are in close contact with an infected person – at least 15 minutes of face-to-face conversation, or two hours in an enclosed space together.

The virus does not appear to be airborne for long periods (one study showed it can survive up to 3 hours in the air but this was under lab conditions). Research is still ongoing into how long it survives on surfaces; the WHO says "for a few hours or up to several days". So far, that same early study has found the virus could last up to three days on hard surfaces like plastic and stainless steel but was less stable on others such as cardboard, fabric and copper. That doesn't mean the virus could infect people for that entire length of time (as the tiny viral particles will begin to decay outside the body). The good news is it can be killed with simple disinfectant (look for the words virucidal or antiviral on the label). Clean your hands with an alcohol-based hand rub or wash them with soap and water. Avoid touching your eyes, mouth, or nose.

People can spread the virus before symptoms appear during the incubation period of between one and 14 days or even if they never get symptoms, making this outbreak harder to contain than others such as SARS, although people are still thought to shed the most virus when they are unwell. WHO spokesman Tarik Jašarević says initial infection rates are constantly evolving, but it is believed each patient with COVID-19 will infect about two or three people. That makes it more infectious than the flu, SARS or MERS but much less infectious than measles, where one person is likely to infect up to 20 people.

In the depths of winter (and flu season) in the northern hemisphere, more people have likely been infected than official counts, although many people with suspect symptoms may have a more common coronavirus or flu.

But the WHO says you can't catch COVID-19 from your pet and it's still considered highly unlikely to spread from food, passing someone on the street, or receiving a package from an infected area. If you are travelling near a confirmed patient on a plane, either in the same row or up to two in front or behind, health authorities say this is considered close contact and you should get advice and self-isolate.

Everyone is urged to wash their hands regularly, keep 1.5 metres away from others and observe good "cough etiquette" by covering their mouth. Chief Medical Officer Brendan Murphy says people must not gather together and should stay home as much as possible but they shouldn't "waste" face masks needed for healthcare workers unless sick or caring for an infected person. Handshakes and kiss hellos should also be swapped for the more chaste pat on the shoulder or elbow bump.

Who has to self-isolate?

To stem the spread of COVID-19, millions of people infected or exposed to the virus around the world are also being urged to quarantine themselves at home, as health authorities turn to detective work to trace contacts of confirmed cases. While some people have chosen to start home isolation themselves, thousands of others have been issued formal notices to comply and many states have flagged hefty fines or penalties for those who break the rules. If you have been in close contact with a confirmed case of COVID-19 or have returned from overseas, then you must self-isolate for a full fortnight from when you landed in Australia. (Overseas travellers will now be put up in mandatory hotel quarantine by governments).

Those with symptoms should seek medical advice (call ahead before fronting up to a clinic) and wear face masks around others to stop the spread – but not everyone in the community with a flu-like illness needs to go to hospital or rush out to get tested. Health authorities say testing kits are limited so are only testing those with symptoms likely to have the virus such as those exposed to a known case.

How does this virus compare to the flu?

Parallels in severity have been drawn with the common flu but the WHO has stressed COVID-19 is much more dangerous, even if influenza is more widespread (between 291,000 and 646,000 people die from it globally each year, according to calculations by the US Centers for Disease and Prevention.)

In Australia, government figures for the most recent season ending October 2019 show 812 people died out of 298,120 reported cases – a fatality rate of about 0.27 per cent. Emerging coronaviruses such as this strain and SARS can also do more damage to the body. SARS punched "honeycomb-like" holes in patients' lungs and such lesions are appearing again in some COVID-19 cases. And, while new vaccines for influenza are developed each season, there is unlikely to be an inoculation against COVID-19 for another year.

It's also possible to be infected by both the flu and the new coronavirus at the same time and so people are being urged to have their flu shots this year. In Australia, health authorities predict coronavirus outbreaks will converge with an early and severe flu season and have reminded the public that 'emergency departments are for emergencies" as hospitals brace for the influx.

What about other outbreaks such as SARS?

It took this new coronavirus 48 days to infect the first thousand people. By contrast, SARS took 130 days and the less infectious MERS 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 across 26 countries but close to 10 per cent of those were fatal. (Most clusters happened in hospitals or households.)

MERS has been circulating for eight years across 27 countries so far, and kills about a third of those who fall ill – out of about 2500 confirmed cases. Ebola is even more deadly, killing more than 40 per cent of those diagnosed over a number of outbreaks since the 1970s but it has been reported in just a handful of countries.

In the pandemic records, the infamous 1918 "Spanish flu" killed about 2.5 per cent of its victims over two years – but because it infected so many people (close to 27 per cent of the world's population) at a time of much cruder medical care, about 50 million died. Today, doctors are much better able to stave off secondary bacterial infections, which proved particularly deadly during that outbreak.

The last pandemic was swine flu in 2009, the second coming of a H1N1 influenza that infected between 11 and 21 per cent of the world's population. Governments mounted costly responses until it was ruled to be over in October 2010. But the virus killed about 285,000 people (fewer than seasonal flu normally does) with a relatively low fatality rate of .02 per cent, and the WHO copped criticism for labelling it a pandemic at all.

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. And MERS also has bat origins but mostly spreads from infected camels, often in slaughterhouses.

Bats are essential to many ecosystems we rely on but, through a lucky quirk in their immune system, they also carry a whole host of diseases that do not harm them. Early work suggests this new virus is 96 per cent similar to a SARS-like strain already discovered in bats a few years back, but experts think it likely first jumped into another animal, possibly the highly endangered and heavily trafficked pangolin, where it gained potency before passing into humans.

China has already temporarily banned its wildlife trade amid the outbreak. But, under pressure from citizens, conservation groups and experts who say the country has not learnt the lesson of SARS, this time Chinese President Xi Jinping is moving to permanently outlaw selling wild animals for food and to further restrict their trade for medicine, scientific research and as pets.

What is shutting in Australia and how big is the risk from COVID-19 here?

The federal government has ruled people shouldn’t gather together at all except for “essential” activities such as school, public transport and work under new social distancing rules likely to last for the next six months. As well as banning outdoor gatherings of more than two people, it has closed indoor venues such as pubs, clubs, casinos, cinemas, gyms and places of worship. Auctions and open houses must stop; and play centres, libraries, community centres, galleries, institutions and more have shut their doors. Supermarkets, shopping centres and businesses such as hairdressers can stay open (under new restrictions) but restaurants and cafes will be takeout only. Indoor gatherings still running should allow space for one person per four square metres but schools will move to online learning after Easter (except for kids who have to attend in person). Weddings can have only five people present, including the couple, funerals 10 people – and house parties could be outlawed.

People shouldn't gather with more than one other person outside unless they are all from the same household and visitors to homes are now mostly banned as states call on their emergency powers to enforce the new restrictions.

Right now in Australia Prime Minister Scott Morrison says you should stay home unless you are:

• Shopping for what you need (such as food or supplies to help keep you at home such as sports mats)
• Getting medical care or for compassionate reasons such as caring for someone
• Exercising outside including dogwalking
• Attending work or school if you cannot attend either remotely

Non-essential access to aged care home has also been cut back to protect elderly people. Those over 70 (as well as people with a chronic condition over 60 or Indigenous Australians aged 50 and above) are now urged to stay home and away from people outside their household, even family, as much as they can, instead leaning on delivery or help to get their shopping and supplies.

Australia has closed its borders to foreign nationals and Australians can no longer travel overseas except in exceptional circumstances. Citizens and permanent residents will now have to see out their 14-day quarantine in hotels, at the government's expense, and those still abroad are urged to come home as other countries shutter their own borders.

Many states are now restricting domestic travel too. Tasmania has already largely closed itself off from the mainland and, in South Australia, the Northern Territory and Western Australia police will patrol borders and ask people entering to self-quarantine for 14 days.

Early cases of COVID-19 in Australia were all linked to overseas travel in China, Iran, the US or on the cruise ship turned floating quarantine site, the Diamond Princess. But more recently, a number of people including doctors and those in aged care, have been infected on Australian soil.

While extra funding has ramped up medical supplies and staff, and opened isolated "fever clinics" to test suspected cases, on the ground many doctors are still wary of a sudden surge in patients as they run low on personal protective gear like masks.

By mid-March, case numbers were doubling in Australia every three to four days but that curve has started to flatten dramatically as the new social rules take effect. Still authorities say now is not the time to "take the foot off the brake". If people do not follow the new social distancing rules, Chief Medical Officer Professor Brendan Murphy warns Australia could lose this containment window and cases could rapidly explode. Authorties have also moved to ramp up testing, amid calls from the WHO to test all suspected cases. While the criteria was strict early on because of a global shortage of testing kits, Australia has high proportionate testing rates compared to many other countries.

We also have one of the lowest positive rates for testing in the world, Professor Murphy says, meaning authorities feel "reasonably confident that we are detecting a significant majority of cases in Australia and that means we can get on top of cases" through contact tracing. That's in contrast to countries such as Iran, Italy and even the US, where Australian authorities say they likely only tested the more severe cases or, as Murphy puts it, "when they detected significant outbreaks they probably had much, much larger outbreaks in community that were undetected".

On April 16, Professor Murphy said new modelling or "nowcasting" using current data suggested Australia was detecting more cases within its borders than anywhere else in the world. He and the Prime Minister have credited the efforts of Australia's army of contact tracers, who work around the clock to hunt down people who have been exposed to a known case of the virus.

But at times, authorities have drawn criticism for letting cases slip through the net, most notably allowing 2700 passengers to disembark in Sydney from the Ruby Princess cruise ship, hundreds of whom later tested positive. But the states have also threatened hefty fines and even jail time for those at risk of spreading the virus who break home quarantine orders.

The government has ordered panic buyers to stand down as stockpiling (and even violent confrontations in toilet-paper aisles) spark fears of shortages, including for important medications such as asthma puffers. Prime Minister Scott Morrison said people can still do their shopping but if everyone floods supermarkets or disregards the new social distancing rules "more draconian measures" will be needed to slow the virus.

A 2019 report into the world's preparedness for a such a health crisis found Australia ranked highly (fourth behind the US, the UK and the Netherlands), even as researchers warned no country was fully prepared for a pandemic.

If you suspect you or a family member has coronavirus you should call (not visit) your GP or ring the national Coronavirus Health Information Hotline on 1800 020 080.

This explainer was originally published on Jan 21 2020.

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