Health authorities are urging Australians to ensure they're vaccinated as whooping cough continues its spread across Australia, with cases in one state hitting a record high not seen in over three decades of records.
It comes as the nation is gripped by an "unusual" convergence of multiple respiratory illnesses, with COVID-19, influenza, RSV and mycoplasma pneumoniae all circulating at elevated levels.
In New South Wales alone, there were 2490 whooping cough cases in July - the largest number in any single month since records began in 1991.
To put that into perspective, that's more than the whole country recorded across the entirety of 2023.
Across Australia, there's been more than 19,300 cases so far this year.
That figure that already rivals the last epidemic year of 2016 which saw 20,117 cases - and there's still five months of the year still to go.
Queensland was the first state to see a sharp spike in cases late last year, followed closely by New South Wales, but other states are now following suit with cases in South Australia and WA surging past pre-pandemic levels.
And health authorities are warning there are likely several more months of this outbreak still to go.
So just what is fuelling such a severe outbreak, and what can we do about it?
What exactly is whooping cough?
Whooping cough, also known as pertussis, is a bacterial infection which starts out similarly to most flu-like illnesses with a fever, cough and tiredness but which can progress into bouts of coughing so severe they cause the elderly to break bones.
More contagious than influenza, measles or COVID-19, once infected a person can remain contagious for three weeks or until they complete a course of antibiotics.
It can be life-threatening in young babies, particularly those who haven't yet been vaccinated against it.
What is causing such a severe outbreak?
It is difficult to say with any certainty, but University of Sydney's Associate Professor Dr Philip Britton believes disruptions from the COVID-19 pandemic are likely to blame.
Disruptions caused by pandemic restrictions and school shutdowns likely had dual impacts: they caused slightly lower uptake of the whooping cough vaccination, while also virtually wiping out the spread of the disease.
The illness typically sweeps the country every three to five years, but COVID-19 restrictions saw numbers drop to near-zero in 2020 and 2021, meaning it's been eight years since the last significant outbreak.
"We probably actually rely on a background rate of transmission of this infection to maintain immunity in the community," Britton explained.
"Many of us were expecting that when (whooping cough) did come back, it was going to be a bigger outbreak than we would have otherwise had."
Medical microbiologist at the Doherty Institute, Associate Professor Norelle Sherry, agrees that the 'immunity debt theory' is likely to blame - and the affects aren't limited to just whooping cough.
"Whooping cough usually peaks in spring and summer, but I think that after COVID we are actually seeing that a lot of these rules of thumb are being broken," she said.
"The lack of exposure to these kinds of pathogens in the community has meant that... we are seeing unusual things like high rates of influenza, RSV and COVID all at the same time."
Just how serious is it, anyway?
Also known as the '100 day cough', whooping cough can be a troublesome illness for someone of any age, with prolonged and severe bouts of coughing impacting sleep and overall health for weeks at a time.
However, it is generally most severe in older people and young children, especially very young babies who have not yet received multiple vaccinations.
Before vaccination for pregnant women was introduced to the National Immunisation Program Schedule in 2018, on average one baby under six months would die and more than 200 would be hospitalised each year in Australia.
While the new push for pregnant women has reduced these figures, it's estimated that around a third of pregnant mothers don't receive the vaccination.
"If you get whooping cough and you're under four weeks of age or you're a premature baby, the risk (of death) is significant in that age group," Britton said.
"Sometimes the whooping cough can even just directly cause them to stop breathing - we call that apnea."
The infection can also spread into the newborn's lungs, causing a type of pneumonia that is particularly difficult to treat.
What can I do to protect my family?
The number one message from infectious disease experts is to ensure all family members are fully immunised.
That includes six pertussis doses throughout childhood at two, four, six and 18 months, followed by one at four years old and a final booster during the first year of high school via in-school programs.
"We've got a larger number of school-aged children than usual who missed out on those boosters, and they're the ones who are showing the highest rate of detection of pertussis," Britton said.
Pregnant women, health care and childcare workers and those aged over 65 are also recommended to get vaccinated, but Britton added that anyone who hasn't been vaccinated in the last five and 10 years could benefit from a booster.
"The risk of whooping cough will extend - my guess is - for several more months, and that is why going and getting your vaccine is going to be of value to many, many people."
How can I tell if my cough is whooping cough?
Telling whooping cough apart from any other cold or flu-like illness can be "tricky", Sherry and Britton agree.
"It's very difficult at the moment because we are seeing increased numbers of all respiratory illnesses, including whooping cough, but also mycoplasma pneumoniae which can also be associated with a severe cough," Sherry said.
Britton described the "classic" manifestation of whooping cough as "a very severe cough - what people sometimes describe as a staccato cough - a very fast, very forceful cough that comes in groups".
This can be - but isn't always - followed by a whooping sound as the person takes a rapid intake of air after a coughing fit.
However, because this distinctive cluster of severe coughing and subsequent "whoop" generally don't start until several days into the illness, the early stages of whooping cough will typically appear like any other cold or flu.
"The best way to tell if it's whooping cough is to do a test, and we particularly emphasise the importance of doing that in young children," Britton said.