The idea is that quarantine protects people from infection by the coronavirus. Quarantine is meant to be a guarantee of health and not the transmitter of illness.
But the authorities in Victoria are struggling to deal with an outbreak which came out of the Holiday Inn at Melbourne Airport where people returning from abroad were quarantined.
The hotel has been deep cleaned and the people in quarantine moved elsewhere.
But the virus is well and truly out.
So what went wrong?
The first thing to be said is that quarantine hotels are concentrations of people who may be infected - that's why those people are in quarantine. The risks are higher than they have been out in the community.
Means of transmission are many, from contact between humans to contact with surfaces contaminated with the virus to people breathing in contaminated air, particularly aerosols (as tiny droplets are called).
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On top of that, the strain of the virus in the Melbourne Holiday Inn is a new and more transmissible one than previous ones.
The authorities believe that this outbreak started with a returned traveller who was using a nebuliser to inhale medication for his asthma. According to accounts he's given reporters, the authorities were aware he was using the device.
What is a nebuliser?
A nebuliser is a small hand-held machine used to administer drugs to a patient. It creates a fine mist of the drug which is breathed in.
Professor Adrian Esterman of the University of South Australia said that an infected person gives out particles of the virus in several ways but usually from the mouth and nose as aerosols. Nebulisers intensify this dispersal of small infected particles, creating air laden with the virus.
"Nebulisers basically create aerosols of medicine for someone to breathe in through a mask. However, they are not airtight and when the infected person breathes out, virus-containing aerosol is dispersed into the room," Professor Esterman told The Canberra Times.
"So, when an infected person sneezes, coughs, sings or even speaks, they give off respiratory droplets containing virus particles.
"Larger droplets can be directly coughed or sneezed into someone's face, getting into their eyes, mouth or nose. They can also contaminate surfaces so that someone can touch the surface, then touch their eyes, mouth or nose and then get infected.
"Smaller droplets containing viruses can hang around in the air for hours and someone can simply walk through them to get infected.
"If windows in hotels can open, then the breeze can disperse and dilute aerosol droplets. Air conditioning can also do this."
Professor Esterman thinks air purifiers might be a good idea. "Instead of dispersing aerosols, portable air purifiers can trap a lot of virus particles in their filters. So, it would make sense to install them in quarantine hotel rooms - another no brainer."
In hospitals, air is circulated frequently, about 15 to 20 times an hour, but in hotels (where quarantined people are being held), Professor Esterman said air is more likely to be refreshed only once to four times an hour "which is simply not good enough to disperse aerosols".
He thinks that if someone is ill enough to need a nebuliser they should be in hospital rather than a quarantine hotel.
Where might the gaps be?
Each outbreak brings new questions. The authorities learn every time from the previous cluster and adapt procedures, learning how to target them more effectively.
The cluster at Melbourne Airport is thought to have started in a family of three who were in quarantine after being infected overseas.
From them, it seems to have spread to the workers in the hotel and then out to the community.
One of the infected people was a catering worker. We don't know how she caught the virus but she was breathing the air which may have been highly charged with contaminated droplets.
Some questions are: what protection do ancillary workers in quarantine hotels have? Do they have masks, for example? Are they kept at a distance from potentially infected people? How often are they tested?
Many, perhaps all, of the originally infected people seem to have been on the same floor of the hotel.
The authorities went through thousands of hours of CCTV footage after the first worker tested positive on February 7 but didn't find any breaches of the rules (like people leaving their rooms and meeting others).
In the absence of evidence of deliberate breaches of the rules, the authorities have homed in on the virus being airborne.
But one traveler tested positive after leaving quarantine
In New Zealand, one person tested positive after leaving 14 days of quarantine. In Sydney, a returned overseas traveler tested positive two days after being released from quarantine at the Sofitel in Wentworth.
One theory is that the infected people went into quarantine without being infected but then caught the virus in the later days of quarantine.
Or they may simply have had a longer period of illness.
"Although the average incubation period (time from being infected to having symptoms) is about six days, some people can take much longer," Professor Esterman said.
"In fact, about one per cent of people have an incubation period longer than 14 days. Hence, by doing a 16 day check, it makes the risk of transmission extremely low.
"The UK is going the other way, with a quarantine of only 10 days!"