On Sunday, there were four cases of COVID-19 in Adelaide.
On Monday, there were at least another 17. Nobody expected that to be the end of the matter.
There's no question that the outbreak of coronavirus cases is serious. The question is how serious.
A determined effort was being made to trace the contacts of the people in the original family who seem to have been the first to contract the disease.
We will find out how serious the situation is over the next few days.
It does seem particularly hard to take because Prime Minister Scott Morrison was upbeat on Friday.
It looked then as though Australians would be able to travel between all states and territories except Western Australia by Christmas.
It may still happen if the situation in Adelaide can be contained relatively quickly but it is by no means certain.
Which direction will it take?
The outbreak may go one of two ways, according to Adrian Esterman, professor of biostatistics in the health sciences department of the University of South Australia.
By Wednesday, it should be clear whether Adelaide is turning into a situation like that in Melbourne or one which is still serious but not so difficult to control like the one in Sydney.
In Melbourne, 655 people died after the virus escaped from two quarantine hotels and ripped through Victoria's aged care facilities. In NSW, there were 55 deaths.
The Adelaide outbreak has similarities to the one in Victoria in that it also seems to have come out of a quarantine centre.
One of the people infected in the first batch on Sunday worked at what is known as a "medi-hotel" for people returning from overseas - ("medi-hotel" is one of those new words to emerge from the pandemic).
The first person thought to have taken the virus into the community is from a big family where other members work in institutions where infections can spread easily - in this case, one family member works in a prison and another in a nursing home. Fifteen of the first 17 cases were all members of one extended family.
"This is very much deja vu. This is very similar to what happened in Victoria," Professor Esterman said.
This is this is very much deja vu. This is very similar to what happened in Victoria.Professor Adrian Esterman, University of South Australia
What are the South Australian authorities doing?
They have started an intensive effort to trace all contacts of infected people.
And more. Professor Esterman said that he was pleased that they were not just tracing contacts but contacts of contacts.
He was hopeful that the outbreak could be shut down but we would find out in the next 24 to 48 hours from lunchtime on Monday.
"I have a lot of trust in our public health system. We have very good contact tracing," the epidemiologist said.
But, he said, he was still "very worried".
The South Australian contact team is much smaller than the one in Victoria, so if the cluster grows, the government might have to call in reinforcement from other states.
"If they crack down on it now, they will be fine. If it gets too much bigger, we'll be struggling to have enough contact tracers," he said.
The health authorities have also created a testing centre in the part of Adelaide where the cluster seems to be concentrated.
About 90 people who were in a hospital emergency department at the same time as one of the infected people were told they must quarantine.
The South Australia Department of Corrections activated a "health rapid response" team to help trace people who might have been infected in the prison.
An Adelaide primary school was closed for a deep clean after a student was identified as a contact of a case. Two more schools would close out of what the education department called "an abundance of caution".
A supermarket was also closed. Staff at quarantine hotels would be tested every week.
How have other places reacted?
Quickly and decisively.
Back in March, South Australia closed its border to the rest of Australia. It insisted that most people arriving from outside the state, including those who usually lived in South Australia, should quarantine for 14 days. South Australia was due to lift its block on Victorian entry on December 1.
Now the situation is reversed.
The Northern Territory, Victoria, Western Australia and Tasmania have all restricted travel from South Australia.
New South Wales and the ACT were waiting to see developments in the coming days. ACT Health said, "If you have been in any of these identified locations and recently returned, be aware of COVID-19 symptoms. If any symptoms develop, no matter how mild, immediately get tested and self-isolate until you receive a negative result."
So it's all gloom and doom?
There are more hopeful signs with the vaccine. It's been announced that Australia is building a large vaccine production facility near Melbourne airport.
The factory will produce flu and fever vaccines for the Australian market and for export.
It's to be built by Seqirus, the flu vaccines division of the Australian company CSL which, as the Commonwealth Serum Laboratories, was established in 1916 to "service the health needs of a nation isolated by war", as the company puts it.
CSL was privatised in 1994 and is now a global biotech business in a market which seems unlikely to shrink.
Seqirus will invest $800 million to create the production facility. The Federal Government will contribute $1 billion over 12 years.
"Keeping Australians safe is my number one priority and while we are rightly focused on both the health and economic challenges of COVID-19, we must also guard against future threats," Prime Minister Scott Morrison said.