Contact tracing is the crucial element in stopping the COVID-19 epidemic spiraling way out of control.
The task is to track down everyone with whom an infected person has come into contact.
It is a race against time - the longer someone who may have the disease stays out there in public places with the virus undetected, the more people he or she can spread it to.
So the army of contact tracers are unsung heroines and heroes of the current battle to keep Australia safe and healthy.
What's it like?
Dr Tambri Housen knows because she's done it and she's helped health authorities across the world devise ways of doing it better.
She was recently in Melbourne helping the effort there.
She said the work was engrossing and even enjoyable.
"It's detective work. No two scenarios are the same. It's putting the pieces together," the researcher from the Australian National University told The Canberra Times.
In police dramas, you sometimes see boards on the wall with names of people of interest and lines and arrows connecting them. It's a bit like that with the disease detectives.
"You can't get past the good old white board," Dr Housen said.
There's a lot of slog tracking people down, and with the imperative of speed. There may be more urgency to this kind of work than there is to a lot of police work where delay may not cost lives.
Dr Housen said that every infected person had an average of ten contacts and they had to be traced quickly.
It doesn't need to be everyone the infected person passed in the street, just people they were in contact with for about 15 minutes.
Sometimes the infected person knows exactly who they were with and sometimes they don't, say, if they were in a pub.
Dr Housen said that was why it was so important for pubs and other venues to take names and contact details of everyone who visited.
What happens next?
If they can, the tracers call the contact up by phone and tell them they've been near someone with COVID-19.
This can be a shock. What's it like for a person to be called up out of the blue and told they have had contact with a COVID-19 case?
"People often want to know who it was but, of course, you can't tell them because it's confidential," Dr Housen said.
This kind of information is kept secret by the health authorities partly because they - and we - want people who might be infected to come forward immediately, and they might not do that if they think their names will be made public.
Once the contact is talked to, he or she is told to go home immediately.
"Then there's the worry. If they are out at work and they have to go straight home and self-isolate, they want to know if they can stop at the shops or pick up the kids," Dr Housen said.
"The advice is: 'No. You have to go straight there'."
The contacted person might or might not be infected but they are not put into complete isolation. They can stay at home with their families but try to minimise contact within the home.
"They are the person at risk - not the family members. The family can go about their business," Dr Housen said.
If, however, the person does get symptoms, the system moves into overdrive and they are tested.
What about homeless people?
Homeless people are a more difficult case because they often don't have a phone or access to a computer.
Dr Housen said that a good method was to use people within the homeless community. It's been done, too, with remote communities where mistrust of outsiders, particularly outsiders in authority, is high.
She has been talking to contact tracers in New York where there are many homeless people living on the streets. The method has worked there. People from within the community speak the same language, both literally and figuratively.
What about the infectious person?
Once a person has been tested, the swab goes to a pathology lab and if the result is positive, the person is called immediately.
According to contact tracers in the ACT, there are a variety of responses, ranging from surprised to anxious. Usually, infected people cooperate.
The caller then goes through the patient's symptoms to try to work out when they were infectious to others.
They then begin retracing the case's steps to find out where they may have spread the virus.
"We really drill quite deeply into that," according to ACT contact tracing team case manager Sue Reid.
"It's not like basic movements, it's more like a day to day, sometimes even minute to minute. You might say to somebody, well were you in a coffee shop? Who did you sit next to? Some of our investigations have been quite protracted."
Are there enough contact tracers?
In Canberra, students in health subjects were trained but the task was manageable because the number of infections was low.
But Victoria is a different order of magnitude. A military team has helped out there, doing follow-up calls with people diagnosed with COVID-19 and with their possible contacts.
"The prompt establishment of a large-scale contact tracing team requires recruitment of the right people, induction, training and coordination in a short space of time," Royal Australian Navy Doctor Lieutenant Commander Anthony Carpenter said.
It's not war as they know it but life and death all the same. Reinforcements may be needed.