Thousands of people could have been exposed to the coronavirus in Australia by the country's nine confirmed patients alone - as up to 49,000 people land on flights from China per week and a global health emergency is declared.

Four people in Sydney, three in Melbourne, and two on the Gold Coast have been struck down with the deadly virus in recent weeks.

Each one flew in from Wuhan, the Chinese city where the outbreak began, on planes packed with hundreds of potential victims.

At least a day passed after they arrived before they developed flu-like symptoms and eventually went to hospital.

Thousands of people could have been exposed to coronavirus in Australia by the country's nine confirmed patients alone

A passenger wearing a protective mask (pictured) is seen at Sydney Airport on Thursday, getting off the last flight to Australia from Wuhan before they were shut down

During this time when they didn't know they were sick, they interacted with hundreds of people each, some of whom could now be infected.

One patient was even allowed out of isolation to celebrate Australia Day at a restaurant with his family while he was waiting for test results.

People they infected will be going about their lives on crowded trains and buses, having business meetings, and sharing meals - potentially passing the virus on.

This is just from the nine confirmed cases - more than a dozen other people are being tested after developing symptoms associated with the virus.

It comes as China's coronavirus death toll hits 170 and the World Health Organisation declares a global health emergency.

Speaking to reporters, WHO Director-General Dr Tedros Adhanom Ghebreyesus said: 'The main reason for this declaration is not because of what is happening in China, but because of what is happening in other countries.

'Our greatest concern is the potential for the virus to spread to countries with weaker health systems, and which are ill-prepared to deal with it.'

Russia, which has no cases of the virus, is closing its 2,600-mile border with China, joining Mongolia and North Korea in barring crossings to guard against the outbreak.

Train traffic between the countries was halted except for one train connecting Moscow and Beijing, but air traffic between the two countries continued.

On Thursday Israel banned all incoming flights from China and 6,000 people in Italy were prevented from leaving a cruise ship while tests were carried out on a passenger from Macau.

Australia and New Zealand are planning a joint evacuation of citizens from Wuhan while Singapore is setting up a quarantine facility on an island to the city's north-east.

Meanwhile, the United States and South Korea confirmed their first cases of person-to-person spread of the virus.

The man in the US is married to a 60-year-old Chicago woman who got sick from the virus after she returned from a trip to Wuhan, the Chinese city that is the epicentre of the outbreak.

Shoppers cover their faces with masks at Asian markets in Cabramatta in Sydney's southwest on Thursday out of fear of coronavirus

A pamphlet handed out by the Australian Government providing travellers with information on the deadly coronavirus (pictured)

In Australia, the first men were confirmed to be infected with coronavirus on January 25 - three in NSW and another in Victoria.

Two arrived on direct flights from Wuhan to Sydney on separate China Eastern Airlines flight MU749, one on January 20 and another on an unknown date.

Both these flights had up to 250 passengers plus several crew on board who are prime candidates for infection.

Another man, diagnosed on the same day, flew in to Sydney via Shenzhen, near Hong Kong, on January 6 on a plane that carries up to 277 people.

CORONAVIRUS CASES IN AUSTRALIA NEW SOUTH WALES: 4 Four people in NSW have been diagnosed with coronavirus, including three men and one woman. January 25 Three men aged 43, 53, and 35 who had recently travelled to China are confirmed to have contracted the disease.

Two flew in from Wuhan while the other arrived in Sydney from Shenzhen, south China.

They are being treated in isolation at Westmead Hospital and are in stable condition. January 27 A 21-year-old woman is identified as the fourth person to test positive for the illness in NSW.

The woman, a student at UNSW, flew into Sydney International Airport on flight MU749 on January 23 and presented to the emergency department 24 hours later after developing flu-like symptoms.

She is being treated in isolation at Westmead Hospital. VICTORIA: 4 January 25 A Chinese national aged in his 50s becomes the first confirmed case of the coronavirus in Australia.

The man flew to Melbourne on China Southern flight CZ321 from Wuhan via Guangzhou on January 19.

He is now in quarantined isolation at Monash Hospital in Clayton in Melbourne's east. January 29 A Victorian man in his 60s is diagnosed with the coronavirus.

He became unwell on January 23 - two days after returning from the Chinese city of Wuhan, the epicentre of the outbreak.

The man was confirmed as positive on January 29 and was subsequently seen by doctors at the Monash Medical Centre. He was assessed as being well enough to stay at home. January 30 A woman in her 40s falls ill with the coronavirus.

She was visiting from China and mostly spent time with her family.

She is being treated at Royal Melbourne Hospital. February 1 A Melbourne woman aged in her 20s is at home recovering. She returned from Wuhan on January 25 and fell ill two days later. She was not infectious on the flight, and has remained in isolation since being tested. QUEENSLAND: 2 January 29 Queensland confirms its first case after a 44-year-old Chinese national wass diagnosed with the virus.

He is being treated at Gold Coast University Hospital. January 30 A 42-year-old Chinese woman who was travelling in the same Wuhan tour group as the 44-year-old man tests positive. She is in Gold Coast University Hospital in stable condition. SOUTH AUSTRALIA: 2 February 1 A Chinese couple were placed in isolation in a public hospital. The man and a woman both aged 60, were from Wuhan and visiting relatives in South Australia. SA Health said the pair isolated themselves at home when they developed symptoms and awaited the results of the coronavirus test. Australia has raised the travel alert level to 'do not travel' for the city of Wuhan - the epicentre of the outbreak - and for the entire Hubei province. Chief Medical Officer Brendan Murphy says unless people have contact with someone who is unwell and has come from that part of China, there is no need for current concern. Advertisement

The fourth arrived in Melbourne from Wuhan via Guangzhou on January 19 on a an A388 carrying up to 525 passengers.

A total of 167 flights that can carry 48,999 people are landing in Australia from mainland China per week as coronavirus spreads around the world.

Based on those figures, as many as 342,993 people may have entered Australia from China since the deadly virus was first detected in Wuhan seven weeks ago.

On Thursday, major airlines suspended or reduced services to China including British Airways, Lufthansa, American Airlines, KLM, and United.

But the airlines that fly from China to Australia - including eight Chinese airlines and Qantas - made no changes to their routes, except for China Eastern cancelling its flight from Wuhan to Sydney last week.

The flights include 62 planes from Guangzhou, 42 from Shanghai and 18 from Beijing per week, with direct routes to Sydney, Melbourne, Brisbane, Adelaide, Perth and Darwin.

On Wednesday, Qantas chief executive Alan Joyce said the airline was considering scrapping its Sydney to Beijing route due to low demand.

The airline had already announced plans to axe that service from March, but Mr Joyce suggested that could happen sooner.

NSW Health advice states that people are at risk of catching the virus if they spent 15 minutes in close contact, such as a face-to-face conversation, or two hours in the same confined space.

Chinese health authorities have confirmed that, contrary to earlier belief, patients are contagious even before they develop symptoms.

On Monday, it was confirmed that a 21-year-old University of NSW student who arrived on a different MU749 flight was infected.

The university told students in an email that she became unwell soon after her flight and isolated herself in her on-campus dorm room for 24 hours before going to hospital.

UNSW is not in semester but there are still some students and staff on campus she could have interacted with, along with on public transport and at the airport.

Many students are terrified they could be infected next, especially with thousands of Chinese classmates due to arrive on campus in coming weeks.

The second Melbourne patient landed on January 21, though it is not clear which flight he was on, and wasn't diagnosed until Wednesday.

The man in his 60s went to hospital two days later but was allowed out of isolation to celebrate Australia Day with his family.

He was with five family members - three adults and two children - at The House of Delight in Glen Waverley between 5.30pm and 7pm.

Victoria's chief health officer Dr Brett Sutton urged other diners to contact the health department and watch for coronavirus symptoms.

Other businesses in the same building as the restaurant on 52 Montclair Avenue have closed until February 9 and others are thinking about shutting up shop.

A cleaner was brought in to disinfect buttons in the building's lift.

One of the patient's adult relatives and one of the children have tested negative so far, and all five are in isolation with him.

The seventh victim has the most capacity to have infected hundreds of others.

The 44-year-old Chinese tourist arrived in Melbourne from Wuhan via Singapore on January 22 and spent several days travelling with a tour group.

On Thursday night it was confirmed that one of them, a 42-year-old woman, contracted the virus herself, while three others were sick.

All eight of his fellow holidaymakers are now in isolation with him at Gold Coast University Hospital.

They all took Tiger Air flight TT566, landing on the Gold Coast about 8pm on Monday on a plane that carries up to 189 passengers.

Queensland Health chief officer Dr Jeannette Young on Thursday said she was concerned about everyone on the plane.

Students at the University of NSW where a classmate was diagnosed with coronavirus say they are too afraid to go back to class

A 21-year-old Chinese student was diagnosed with coronavirus earlier this week after she flew back from Wuhan on January 23

'My concern is that those 150-200 people on that plane when he started getting symptoms and then his 24 hours in the Gold Coast community, I need to track exactly where he went,' she said.

Tiger Air said it was in the process of contacting the passengers and crew to notify them so they could visit their GP for testing.

The 44-year-old stayed at an apartment in the Oracle building in Broadbeach, which is in the same building as the $900-a-night Peppers hotel frequented by celebrities including Taylor Swift.

He became increasingly unwell after first developing symptoms before the flight out of Melbourne and called an ambulance about 3.30pm on Tuesday.

A Chinese tourist who arrived in Melbourne early last week was diagnosed on Thursday a week after she got sick.

Her exposure to the public is believed to be more limited as she spent the vast majority of her time with family members she was visiting.

Coronavirus victims' journey to Australia: What we know Queensland's two confirmed coronavirus cases are a Chinese nationals - a man and a woman - from the same tour group that spent about six days in Melbourne before flying to the Gold Coast. The man, 44, fell ill on Tiger Air flight TT566 from Melbourne to the Gold Coast on Monday night. He was travelling with eight other tourists, four of whom have fallen ill and were being tested for the virus. A 42-year-old woman - who was part of the same tour group - was confirmed on Thursday night as Queensland's second coronavirus case. WHAT WE KNOW * The tour group came from Wuhan and stopped in Singapore before flying to Australia. *They left Singapore on either January 21 or 22, and landed in Melbourne on January 22. *The group spent the next five days in the Victorian capital touring the city. *Authorities are now trying to track their movements and alert people who may have been in contact. *The tour group boarded Tiger Air flight TT566 on January 27 at 7.44pm AEDT and landed at the Gold Coast Airport at 8.45pm AEST. *The passenger was travelling on a booking with one other person and they were seated in 11A and 11B. *The 44-year-old developed symptoms on the flight, which was carrying about 150-200 passengers. *The man travelled to his Gold Coast hotel, before calling an ambulance. He spent less than 24 hours in the hotel. * The man and four others from the tour group who developed symptoms have been isolated in the Gold Coast University Hospital. Advertisement

An man infected with coronavirus visited The House Of Delight restaurant in Glen Waverley, Melbourne's south-east, with his five family members between 5.30pm and 7pm on January 26

Other businesses in the same building as the restaurant on 52 Montclair Avenue have closed until February 9 and others are thinking about shutting up shop

LATEST AUSTRALIAN FIGURES ON THE NOVEL CORONAVIRUS OUTBREAK Nine people have contracted the virus after travelling to Australia from Hubei province in central China. * The tally is four in NSW, three in Victoria and two in Queensland. Two of the NSW victims are no longer infectious and were discharged from hospital on Thursday. The others are all stable. * The latest case is a 42-year-old Chinese woman who travelled from Wuhan to the Gold Coast via Melbourne. She is now in isolation in a Gold Coast hospital. * China's women's soccer team, which includes 32 players and staff, remains in isolation at a Brisbane hotel after arriving on Wednesday. * The federal government plans to evacuate Australians from the Hubei capital of Wuhan after about 600 citizens and residents registered for help to get out. China must approve the plan. * They will be flown to Christmas Island and put in quarantine for two weeks. * The federal health department has issued new advice amid evidence the virus has been spread by people before they develop any symptoms. * Anyone who has recently been to Hubei or had contact with a confirmed case must isolate themselves at home for 14 days from the date of their departure. THE WORLD: * Some 170 people have died from the virus in mainland China, and the number of confirmed cases is now more than 7000. Most of the deaths have been in Hubei. * Some major airlines have halted all flights to China, but so far Qantas is not among them. * The virus is now affecting most of Australia's regional neighbours including Singapore, Malaysia, Taiwan, Japan, Vietnam and Cambodia. TRAVEL WARNINGS: * The Australian government is advising people to reconsider travel to China and not go to Hubei province. (Sources: China's National Health Commission, Australian Chief Medical Officer and state chief health officers) Advertisement

People the nine patients infected could pass the virus on to others before they even know they are sick or were exposed at all.

Scientists are still trying to fully understand this strain of coronavirus and work out how contagious it is and how to stop it.

The infection rate of diseases is called its basic reproduction number, known as R0 or r-nought, brought to public attention by 2011 film Contagion about a fictional worldwide pandemic.

Imperial College London estimated coronavirus' R0 to be 2.6, meaning that on average each patient would infect 2.6 others before they died, were isolated, or got better.

This number changes over time as the virus spreads or is brought under control by health authorities - an R0 of less than 1 means it is under control.

Paramedics wearing Hazmat suits arrived at Peppers, on Elizabeth Street, Broadbeach, to reports of a suspected coronavirus case on Tuesday

The luxurious 4.5 star hotel is a hot-spot for celebrities, and is understood to have hosted popstar Taylor Swift and, then boyfriend, Tom Hiddleston in 2016

Measles is particularly contagious, with a score as high as 16, while SARS was 2 to 5, as is HIV, while a standard flu is about 1.3.

Some particularly deadly diseases, like ebola, have low scores because they kill patients too fast to infect many people, or produce symptoms too fast.

Coronavirus has a mortality rate of about 2 per cent, compared to 9.5 per cent for SARS and 34.5 per cent in the 2012 MERS outbreak.

Australia is starting at a low base of infection, but this can quickly multiply if the average R0 observed in China is met.

Coronavirus has killed 170 so far, all in China, and more than 7,800 are infected worldwide.