Details surrounding the initial source of Adelaide's coronavirus cluster have been revealed as South Australia races to contain its current Covid-19 outbreak.
Genomic testing has found the mystery source is a traveller who came into Adelaide on November 2 and tested positive the next day.
South Australia's chief health officer Professor Nicola Spurrier confirmed the traveller's virus strain matched that of the first confirmed case of the Parafield cluster – an 80-year-old woman.
"We've tested her strain and we've linked it to somebody in the medi-hotel where her daughter worked," Spurrier said.
"The person who was the traveller arrived in South Australia on November 2 and was tested on November 3. So, prior to this, prior to November 2, we did not have COVID in the state, but that's when it was introduced, this particular strain."
Around 4000 people are now in quarantine in the state who are now considered close contacts of confirmed cases.
And at medi-hotel Peppers Adelaide, quarantining guests screamed from their balconies on Tuesday afternoon, devastated they will have to undergo another quarantine period, despite testing negative for coronavirus.
Meanwhile leading Australian epidemiologist Dr Emma Miller has warned Australians just how infectious Covid-19 is, saying if you come close enough to be exposed to it, "you're going to get it".
"Everyone that comes in contact with this is susceptible," Miller, a senior lecturer from Flinders University, who is based in Adelaide, told news.com.au.
"If you come in contact with it and are exposed, you're going to get it."
She said the real question for South Australians was how much time – or "circulation" – the virus had in the community between the initial infection occurring and the first case arising.
Miller said South Australia is "very much on the precipice" and there is potential the state could see this cluster turn into a second wave if the virus continues to spread.
Burnet Institute epidemiologist Professor Michael O'Toole echoed those comments, describing South Australia's outbreak as "kind of a microcosm of the beginning of the second wave in Victoria".
Miller said she understood the idea of quarantine is "scary" but defended public health officials, saying the rules were in force due to an "abundance of caution".
There are now 20 confirmed cases linked to the Parafield coronavirus cluster and another 14 suspected cases who are showing symptoms and are close contacts of the already confirmed cases, SA Health said. Those suspected cases are either waiting for test results or have had a negative test result but are being retested.
"These are people who are particularly younger children who have tested negative but may have symptoms and have a parent that's positive," Spurrier said.
"We're being extra, extra cautious."
South Australia recorded five new cases on Tuesday – one who is an aged care worker and three family members of a security guard who worked in the medi-hotel. The fifth case has been confirmed and the person is being interviewed, while the ages of the cases range from their teens to their 50s.
"That's what the problem with this virus is," Miller told news.com.au.
"This is the problem with any pandemic virus – you're talking about a novel virus which nobody has any resistance [to] whatsoever.
"If you come in any meaningful way in contact with this virus and you don't have any innate immunity or haven't had it before, you're going to get this virus.
"There are 7 billion people on this planet and that's the problem; that's why we have a pandemic.
"The fact that we're all susceptible means that, while most will have a mild form of the disease, in absolute terms we're getting a lot of deaths and a lot of serious complications as a result."
The cluster has triggered closures and isolation warnings across the state with five schools and an aged care home now shut for deep cleaning.
More than a dozen schools, hotels, cafes and supermarkets have been linked to the cluster and 4000 people who are considered close contacts of the confirmed case are now in quarantine.
Health investigators earlier said a cleaner became infected via a surface at the quarantine hotel then passed the virus on to two security guards and members of their extended family.
"We've known this about all the respiratory viruses," Miller said.
"In fact most colds can be spread by fomites – the bug which is left on surfaces – and people touch those surfaces and then touch their eyes normally. It's a very common way of spread.
"We know how long this particular virus lives on surfaces and we know that it doesn't stay long on paper, cotton, but it does stay for a long time on materials like steel and plastic.
"Most respiratory infections in cold and flu season can actually be spread that way. This is why we tell people to wash their hands, to not touch their eyes – that's the easiest way not to spread respiratory infections."
More than 6000 tests were expected to be completed on Tuesday and the state is on track for record testing numbers for a single day.
Yet Miller was confident the state health department was "swinging into action".
"It sounds scary, but it's actually not yet that bad," she said.
"The new cases that have arisen are mainly attached to that original cluster. We're still not seeing any large scale community involvement."
On Monday, South Australian Premier Steven Marshall issued advice to residents to wear masks on public transport and where it's not possible to socially distance.
He said his "unequivocal priority" was keeping the people of the state safe and strong but noted "time is now of the essence and we must act swiftly and decisively".
"We cannot wait to see how bad this gets."