"He didn't think he was sick": Everything to know about the COVID-19 'super spreaders'.

It all began with roughly six colleagues sharing a meal...

After days of creeping diagnoses, the source of the cluster of COVID-19 infections in the south-western Sydney suburb of Casula has been identified.

Virus detectives have concluded that a Melbourne freight worker, who travelled to NSW on June 30, passed the virus to a number of colleagues during a visit to their Sydney office. The group then went to a dinner party at Casula's Crossroads Hotel on the night of July 3.

It's believed two to three of the people at the party were infectious. 

NSW chief coronavirus detective, Jennie Musto, said that the man didn't think he was particularly unwell.

"[He] didn’t think he was sick with COVID. He travelled on the 30th of June — he’s been in NSW for a while," she told reporters yesterday.

"And it wasn’t until we interviewed him and his colleagues with more detail that we made the link that they were all at the Crossroads on the 3rd of July."

Speaking to the media this morning, NSW Chief Health Officer Dr Kerry Chant said now "around 40" confirmed COVID-19 cases have so far been linked to the party.

It's a sadly familiar story. 

We are urging anyone who visited the Crossroads Hotel, Casula from July 3 to July 10 to come forward for testing & self-isolate for 14 days. The next 2-4 weeks are critical for NSW. pic.twitter.com/2ODU47haxE

— Gladys Berejiklian (@GladysB) July 12, 2020

Throughout the pandemic, there has been a number of events in which one or a few individuals have been responsible for a disproportionately large number of infections.

In South Korea, roughly 40 people who attended a single church service were infected with COVID-19 at the same time; at least 52 people tested positive after attending a choir practice in the US state of Washington; and 71 people were infected in an apartment building in China after an asymptomatic woman returned from the US and used the communal lift.

Here in Australia, a wedding in the NSW coastal suburb of Stanwell Tops in March saw 35 guests infected by a group that had recently arrived from the US. And of course, there was the Newmarch House aged-care home outbreak, in which the diagnoses of 34 colleagues and 37 residents were traced to a single staff member. Tragically, 19 of those residents passed away.

The source of these cases is known as 'super spreaders'.

Super spreaders aren't unique to COVID-19. Some researchers estimate that, in any given outbreak, 20 per cent of the population is usually responsible for causing over 80 per cent of all infections.

Which is precisely why identifying them and their contacts — and doing it quickly — is crucial to halting the spread of the disease.

Why do some people become 'super spreaders'?

There are a variety of reasons that a person with COVID-19 may infect more people than is typical. 

Writing for The Conversation, Professor Elizabeth McGraw, an infectious disease specialist from Pennsylvania State University, points to these key ones: the individual's behaviour, the pathology of the disease, and the individual's biology.

Behaviour is the obvious one: think travel patterns, the degree of contact they have with others and with commonly touched surfaces.

That's not always a matter of carelessness or selfishness, though. 

10 new cases of #COVID19 have been diagnosed in NSW between 8pm on 14 July and 8pm on 15 July.

For the latest list of COVID-19 locations, visit: https://t.co/pqkRdfh3cR pic.twitter.com/XKy3TphHOs

— NSW Health (@NSWHealth) July 16, 2020

A recent review of data from nine international studies suggest that roughly 15 per cent of people with the virus don't show symptoms, and there's evidence that some others can transmit the virus in the pre-symptomatic phase. In other words, they spread the virus inadvertently. (There's the pathology part.)

Plus, it can also come down to a person's biology: "Some infected individuals might shed more virus into the environment than others if their immune system has trouble subduing the invader," Prof. McGraw states.

"This is a critical period": The characteristics of the Casula cluster.

At least four new cases have been linked to the Crossroads Hotel outbreak in the past 24 hours.

Virus detectives are now working to determine if three other cases in NSW, currently of unknown origin, are also linked.

And they're working particularly feverishly. 

Chief Health Officer Chant yesterday told media that, in several of the cases linked to the outbreak in Casula, those infected had developed symptoms in just a day or two.

The incubation period — that is, the time between contracting the virus and developing symptoms — is typically between 1-14 days. So these cases have been developing incredibly fast.

"It gives you very little time for the contact tracers, because you've got to get your cases diagnosed and then you've got to lock down those contacts," Dr Chant said. 

"If you've got a sore throat today, don't wait for two days to get it diagnosed. Work with us. Go get tested day one because every day you can give us allows us to stop that spread.

"I am far from relaxed," she continued. "This is a critical period for us and I need the co-operation of the public."

Featured image: Getty.