They're two words that have quickly become part of the everyday language as people adapt to living with the threat of coronavirus: community transmission.
Making sure it's kept down to as low as possible, has been a critical part of Australia's COVID-19 response.
It's because of large levels of community transmission in places such as Victoria that has led to weeks of lockdown measures in the state.
Of particular concern are cases of community transmission unwittingly starting new virus clusters.
Health experts have said stopping further instances of community transmission in virus hotspots will be critical in order for some aspects of life to return to normal.
So what is it?
Just because a person has become infected with coronavirus does not automatically mean it has been a case of community transmission.
Community transmission refers to a case of coronavirus where it was acquired in Australia but the source of infection hasn't been able to be found.
Meanwhile, a locally-acquired case refers to a coronavirus case where it is known how the person became infected with coronavirus.
Infectious diseases expert and associate professor at the Australian National University, Sanjaya Senanayake, said a large amount of cases recorded in Australia during a second peak of infections can be classified as community transmission.
He said it was far-removed from how the virus spread during the initial stages of the pandemic in Australia.
"The early part of the outbreak distinguished itself from community transmission when someone came from overseas and gave it to someone else in the community, as opposed to someone in the community giving it to each other," Professor Senanayake said.
"Two-thirds of cases in the first surge were linked to overseas travel.
"It's a completely different type of outbreak now. What we have seen with Victoria is that the virus established itself in the community."
Figures from the federal government show that as of September 7, almost 5000 of the nation's 26,000 coronavirus were locally acquired with the source not identified, or roughly 19 per cent of total virus cases.
Professor Senanayake said those unidentified case represented the greatest concern to health authorities in trying to stop the spread of the virus.
"One unknown source is a worry because if that person is unidentified and not self-isolating, they could be spreading the virus to others," he said.
"This virus has a tendency to cause a super-spreading event.
"You only need one person in the wrong place in the wrong time and you have a big cluster."
Why is the situation so bad in Victoria?
The sheer number of community transmission cases that were circulating in Victoria gave authorities in the state little choice but to enact lockdown measures. At the beginning of July, Victoria overtook NSW as being the jurisdiction with the most amount of community transmission cases recorded.
It's a completely different type of outbreak now.ANU infectious diseases expert Sanjaya Senanayake
While the number of new daily cases in the state has been dropping in recent weeks, there are still several cases linked to community transmission.
Professor Senanayake said the large number of cases seen in Victoria, where as many as 700 were recorded in 24 hours, made it difficult for contact tracing teams to get to the bottom of the source of infection and prevent community transmission cases spreading further.
"On average, each of those new cases have 10 close contacts, so if Victoria had 700 new cases in a day, in a very short time, the amount of close contacts expands," he said.
"They had to go into lockdown so even if [health authorities] couldn't identify the source, those who had the virus would be at home.
"The numbers are coming down, and certainly that fits in with the measures that are in effect."
How do you prevent community transmission?
According to Professor Senanayake, NSW should be looked to in how to manage community transmission cases in Australia.
While a number of clusters have popped up in areas across Sydney, some linked to community transmission, Professor Senanayake said health authorities had managed to keep a lid on the virus spreading by acting quickly to remove any doubt.
"Generally, public health authorities have identified cases in a timely manner and then self-isolate and quarantine them," he said.
"Remember, we still don't have a magic pill that prevents us from getting coronavirus and we still don't have a vaccine, so the easiest way to stop community transmission is to identify cases and close contacts as soon as possible and quarantine them."
Other measures have been undertaken to better understand where unidentified cases of coronavirus may be spreading and where community transmission could be occurring.
Among them has been testing of the sewerage systems in major cities for traces of the virus in the event some cases may be flying under the radar of health authorities.
Professor Senanayake said in many instances, a small number of people were responsible for a majority of the transmissions of coronavirus. He said caution while going into areas where large numbers of people were gathering like cafes, restaurants and bars was essential in the event someone at the venue was unknowingly carrying the virus.
"Elimination of coronavirus by its definition is possible, but the virus can be reintroduced, and that is the challenge that we're facing," Professor Senanayake said. "Authorities have to be careful about minimising the possible gathering of people.
"It's not only just a person that is infectious, but it also depends on the type of environment that that person is in."
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