It's the Covid variant that has forced Australia's largest city into almost one month of lockdown, placed all states and territories on high alert and the country into its most precarious situation of the pandemic this year.
The Delta variant is the predominant strain of Covid that has been circulating in the community in Sydney's suburbs, before it spread into other parts of the city and forced all of Greater Sydney into lockdown.
Cases linked to the Delta variant have been detected in Melbourne, Brisbane and other cities across the county, all linked with the Sydney outbreak which has had more than 100 cases a day in some instances.
Health experts and epidemiologists have expressed concern about how fast the Delta variant can spread to other people, as new restrictions are imposed around the country to help prevent new infections.
Here's what you need to know.
What is the Delta variant?
Officially known as B1.617.2, the Delta variant was previously known as the Indian variant, after it was first detected on the sub-continent earlier this year during the country's dramatic spike in Covid cases.
Its name was changed to the Delta variant following new guidelines from the World Health Organisation on naming new strains of Covid based on the Greek alphabet, rather than their country of origin.
The WHO has said Delta was becoming the most dominant variant of the virus that was circulating, with its director-general stating it was "the most transmissible of the variants identified so far".
Since it was first detected in India, Delta has been found in more than 90 countries across the world, including Australia, where it has led to the Sydney lockdown.
Why is the Delta variant so concerning?
What has worried health officials is how quickly the variant can spread from person to person.
Of particular concern was one incident in Sydney's Bondi Junction Westfield, where a person in one of the stores contracted the Delta variant after a "scarily fleeting" contact with another person who was already infectious, NSW's chief health officer Dr Kerry Chant said.
Data has indicated the latency period, the time between when someone gets infected with the virus and when they become infectious to others, is shorter for Delta than other variants.
Infectious disease expert from the Australian National University, Professor Peter Collignon, said the variant still spreads in the same way as the original virus that came out of Wuhan, or other variants that have come since.
"This variant seems to make people infectious for a day longer, and it may be that [the variant] latches on to receptors with more firmness and has the ability to multiply more," Prof Collignon said.
"It still spreads in the same way and that's when you're in an indoor environment and close to others and it spreads predominantly through droplets.
"The Delta variant is a concern because it spreads more easily than the original or the UK strain."
Canberra Hospital senior specialist in infectious diseases, Dr Sanjaya Senanayake, said the number of people Delta could spread to from one person was also a concern.
"For the original strain, on average one person passes the virus on to 2.5 people, but with Delta, one person infects about five people," he said.
"Data from the UK suggests it's at least 40 to 60 per cent more transmissible than the Alpha strain."
However, while Delta may be able to spread more easily than other variants, it is still not yet known if it is more virulent than other strains.
How did Delta track overseas?
The variant was first detected in India earlier this year, which led to a surge in new infections and deaths in the country.
Prof Collignon said a lack of facilities in India made the outbreak of Delta there worse than it was.
"It was a resource-poor country with poor housing compared to us," he said.
"The more disadvantaged [a country] is, the more it is knocked around by Covid."
The Delta variant has become the predominant strain of Covid across many countries around the world since it was first identified.
It had overtaken the Alpha, or UK variant, in Britain in terms of the number of new infections, with Delta making up more than half of all new cases there.
America's Centre for Disease Control and Prevention said earlier in June it was making up about 10 per cent of new cases and was now responsible for many large outbreaks across the country, despite high levels of vaccinations.
How is the variant against vaccines?
There is some good news to come out of the Delta variant outbreak in Sydney: Covid vaccines are highly effective against the new strain.
Prof Collignon said in the UK, where the Delta variant has been spreading fast, evidence has suggested the available vaccines - Pfizer and AstraZeneca - were effective against Delta.
"With two doses of either Pfizer or AstraZeneca, they are both equally good at reducing hospitalisations and deaths. The whole point of vaccinations is to stop people dying or falling seriously ill," he said.
"For deaths, it was 95 per cent or more effective, and 90 per cent or more effective at stopping people from going into hospital with Covid, and that means vaccines are the answer.
"For things such as deaths or hospitalisations, these vaccines have been good against the Delta strain."
Will this lead to more new variants?
The nature of the virus will mean that more variants will be a factor as the pandemic continues.
Already, a variant of Delta, known as Delta plus, has been detected in India, with several hundred cases already identified.
Prof Collignon said in many cases, new strains of the virus may not be as virulent, or as serious, as previous versions, but may be more contagious.
"The short answer is that yes, it will lead to new variants, but one would hope that it would cause less deaths, because in the interest of the virus, it doesn't want to kill too many people if it wants to spread," he said.
"With all of the variants, a number will increase in transmissibility.
"However, no matter what the strain is, the data shows two doses of the vaccine is effective at reducing the risk of death or hospitalisation."
Dr Senanayake said increasing vaccination rates were essential to prevent more dangerous strains of Covid from circulating.
"If there are lots more infections, there could be some strains that may make Delta look tame," he said.
"If there are continued cases around the world in large numbers, the virus could then mutate into something worse in terms of hospitalisations and severity of illness."
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