As public health and infectious disease experts devise strategies to guard against a repeat season of similar proportions, some flu sufferers may be wondering why they got away with a few sick days and sore throats when others were killed by the viral infections. 

More than 180,000 flu cases were confirmed nationally by the end of September. That is 2½ times the number of cases over the same period in 2016, though the comparison may be overblown due to the rise in rapid flu testing this year.

Between January 1 and September 15, 288 influenza-related deaths were reported to NSW Health or logged with the Registry of Births Deaths and Marriages. It is likely to be an underestimate. Only a small fraction of all flu cases are laboratory-confirmed and there are delays in updating the register.

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A suboptimal flu vaccine that offered as little as 15 to 20 per cent protection hampered efforts to stem the spread of the viral infection.

While the vast majority of flu deaths every year are among the elderly with pre-existing conditions, the young and healthy are not immune.

Three children, including three-year-old Sydney girl Vanika Idnani, died with flu in NSW this flu season. A handful of young, fit adults die flu-related deaths each year in Australia.

But what exactly are people dying from?

Death by influenza is complicated, said Dominic Dwyer, medical virologist and infectious diseases physician at Westmead Hospital, and clinical professor at the University of Sydney.

The frail and elderly are not the only patients at risk of dying from influenza. The viral infection itself can be fatal to a small proportion of otherwise healthy people or can trigger a fatal secondary infection.

Every flu season intensive care units treat a small number of people who have an over-exuberant, uncontrolled immune response to influenza, Professor Dwyer said. 

"They have this overwhelming reaction to the infection and their lungs fill up with fluid, and otherwise healthy people can die," he said. 

It is unclear why these patients have this terrible response, though it may be a genetic predisposition.

Patients are drowning in their own secretions, Professor Dwyer said. 

Their lungs fill up with fluids containing proteins that fight infection, immune cells and some with antiviral and anti-inflammatory properties. 

"The body produces these proteins to fight the infection. Normally these things naturally operate with a feedback mechanism; when the infection goes down they also drop back, but for some reason in these over-exuberant cases it just keeps building up," he said. 

The syndrome not only attacks the lungs but can cause kidneys to shut down and affects the heart. 

On chest X-rays the lungs of these patients look like a complete "white-out", Professor Dwyer said. 

"It's a terrible disease, and fortunately it's not common," he said. 

Most people who die flu-related deaths are not killed by an influenza infection itself, but by secondary infections, most commonly pneumonia. 

Staphylococcus and pneumococcus bacteria colonised harmlessly in the throat and nose can break through damaged mucosal lining of the respiratory tract caused by influenza, Professor Dwyer said.

This flu damage provides a "portal of entry" for the bacteria to get into the lungs or bloodstream, becoming harmful infections.

Damaged lungs caused by flu also leave patients vulnerable to staphylococcal or pneumococcal infections.

"The bacteria grows and replicates in the lungs, clogging them up," Professor Dwyer said. 

Pneumonia usually affects part, or a lobe, of a lung or both lungs. On an X-ray the white cloud is usually localised to the affected area. 

Secondary infections are most likely to be fatal in patients with underlying respiratory conditions whose lungs are already under strain.

"Add a secondary infection over the top and they can get dramatically worse," Professor Dwyer said. 

People with asthma, cystic fibrosis, chronic obstructive pulmonary disease as well as former smokers were at higher risk of contracting pneumonia.

Even other diseases that don't affect the lungs, including diabetes and kidney failure can get worse and potentially fatal with the added strain of flu.

"Every winter we see heart attacks and strokes increase during the flu season and we know flu increases the risk of both," Professor Dwyer said. 

In rare cases, otherwise healthy people with flu can die of pneumonia. Rarer still, healthy people die of secondary infections that cause inflammation of the heart or brain, he said.

Children die of flu from the same conditions that affect adults, and children with underlying respiratory conditions – like asthma – or heart problems were at increased risk, Professor Dwyer said. 

Roughly half of the children who died from with influenza had no known underlying condition, said infectious diseases paediatrician Professor Robert Booy.

"The majority of flu deaths in children are before they get to school age," said Professor Booy, head of clinical research at the National Centre for Immunisation Research and Surveillance in Australia.

Flu and other viral respiratory infections are known risk factors for meningococcal disease, Professor Booy said. 

Like staphylococcus and pneumococcus bacteria almost everyone will carry harmless meningococcus bacteria in their nose and throat at some stage in their lives. 

The bacteria "invade" a child's bloodstream, becoming a harmful infection via the damaged lining of their throats caused by flu, Professor Booy said. 

But flu and meningococcal deaths among children were still rare, he said. 

smh.com.au