Most likely it arrived from Europe with soldiers returning from the battlefields of France and Belgium, a silent stowaway who would soon wreck havoc on the Australian population and leave as many as 15,000 dead.
This was the Spanish Flu, an unusually virulent strain of pneumonic influenza that in 1918-19 would kill as many as 100 million people around the world, far more than the number left dead in World War I.
The first case was detected in Australia in January 1919 in Melbourne, indicating its arrival aboard one of the many ships returning from Europe bringing home Australian soldiers and also their new wives (international air travel was still in the future).
Many other cases immediately followed and people started dying as the disease spread across Australia.
Australian authorities were aware of the growing pandemic in Europe in 1918 and dared hope that enforcement of strict quarantine could keep Australia flu-free.
Quarantine restrictions were imposed on incoming ships on October 17, 1918, with the first flu case detected the following day. In six months, more than 300 ships were intercepted, with better than half transporting passengers with the flu.
Federal and state health ministers and officials divided up responsibilities, with the states providing hospital and health services and the Commonwealth organising maritime and land quarantine and declaring affected areas.
The new Commonwealth Serum Laboratory set about developing a vaccine, eventually producing three million doses.
That targeted dangerous secondary bacterial infections and was later judged to have been at best partly effective.
But given the number of returning soldiers, it was probably a forlorn hope that Australia could remain flu-free. It wasn't just the returning soldiers but their new wives - by November 1918, Australian soldiers were marrying English women at the rate of 200 a month.
For all the precautions, the scale of the pandemic still caught authorities by surprise.
To contain it, quarantine stations were installed at state borders and isolation depots in cities.
"All public life was controlled. Streets were sprayed with disinfectant and the use of public transport was restricted," historian Joan Beaumont wrote in Broken Nation - Australians in the Great War.
"Schools, theatres, dance halls, churches and hotels were closed, although Anzac day was still celebrated, particularly by returning soldiers. When in public people were required to wear masks."
Perhaps 40 per cent of Australia's then population of five million caught the flu. Onset was rapid, with some reportedly feeling well in the morning and dying later that day.
Unusually among seasonal flu strains, which generally hit the old, young or unwell hardest, a large proportion of Spanish flu fatalities were among healthy men aged 20-40 and pregnant women.
Quite why still isn't completely clear but modern research suggests the flu may have sent their healthy immune systems into overdrive.
Whatever the reason, the consequence was that young men who had survived the trenches died, as did young men who had never set foot outside Australia.
The Spanish flu wasn't Spanish. It's been suggested that wartime censorship suppressed reports of outbreaks elsewhere in Europe but not in neutral Spain, which was thought to be especially hard hit. The truth was it wasn't but the name has stuck.
There are many theories as to its origin - China, France and even rural US state of Kansas. One theory suggests it originated on the Western Front, a consequence of mutations caused by chemical weapons such as mustard gas.
Whatever its origins, the circumstances of the Great War proved ideal for its spread, with huge numbers living in unhealthy conditions in close proximity to each other and then dispersing throughout the world once the war ended.
No corner of the globe escaped, with outbreaks on remote Pacific islands and in the outback of Alaska.
As badly as Australia was hit, we were far better off than many other nations, indicating the government's precautions weren't a wasted effort.
Nine thousand died in New Zealand in two months. The death toll in third world countries, where conditions were frequently more crowded and less sanitary, were orders of magnitude higher.
Like everywhere else, the Spanish flu hit Australia in two waves.
The first wave infected many but didn't kill all that many. The second wave, a more virulent mutation, proved far more deadly and produced the greatest death toll.
This second wave was well under way in Europe and the US in late 1918 - October 1918 was the deadliest month - and that prompted Australian authorities to take the measures they did.
Early cases in Australia were mild, so mild that authorities doubted it was the Spanish flu, but deaths soon followed. Remote from the east coast and with strict state border quarantine in place, Perth wasn't hit until June.
By the end of the year it was all over. The pandemic had simply exhausted itself, leaving behind a nation grieving for their more recent dead, as well as the 62,000 dead from the Great War.