Kelly Clarke died of pneumonia on her honeymoon. How does this happen?

Sydney newlywed Kelly Clarke was honeymooning with her husband Chase in Fiji when she fell ill on Wednesday and was rushed to hospital.

Two days later the 24-year-old nurse was dead.

Her death has devastated her husband, family and friends and has left the rest of us wondering how this could possibly happen to someone so young.

Chase has told the Daily Telegraph that her official cause of death on Friday was determined as severe bilateral pneumonia caused by a fungal infection. He said there were an additional four "minor" causes highlighted.

"When we're stronger we will reveal the complete causes of death but severe bilateral pneumonia is the main one," he said.

So what exactly is severe bilateral pneumonia?

Dr Brad McKay explained to Mamamia that pneumonia occurs as a result of a bacterial, viral or fungal infection, with fungal being the rarest form.

"If it's bacterial, it causes puss, which takes up space in your lung. So instead of having air flowing in and out and oxygenating your blood, it's filling up that space with puss," the Sydney GP said.

"There's also inflammation, which causes more fluid, and as the lung becomes more inflamed you get more fluid on board and that takes up space as well.

"All of which makes it difficult to breathe."

He said if the infection is caused by a virus, such as the flu, the inflammation and fluid is the problem.

"Bilateral pneumonia is where you've got both of the bases of your lungs filled up with fluid or puss... and then as you've just got half of your lung able to transfer oxygen, you're going to be breathing twice as fast."

"If that goes on long enough, your muscles start to tire and then if you're not given further medical treatment, the muscles around your lung will tire out and it will get harder and harder to breathe."

Dr McKay says pneumonia can be a complication of the influenza virus, which pregnant women, people who are overweight and those with a weakened immune system, are of greater risk of developing.

However, he says pneumonia is "really unlikely" to kill a healthy, young person if it is effectively treated with antibiotics and any other in-hospital treatments.

The Daily Telegraph reports Kelly was diagnosed with the chronic immune disorder lupus earlier this year. A common treatment for this is steroids, that can weaken the immune system and increase the risk of infection.

Certainly, this would make sense as the young woman's pneumonia was the result of the rarest cause - a fungal infection - which Dr McKay says would normally be fought off by a healthy immune system.

Adding to the confusion over her death, however, is the fact that she first was diagnosed with typhoid, after presenting to hospital fearing she had a stomach bug.

Common typhoid symptoms are diarrhoea, nausea and vomiting - and Dr McKay said it's rare for typhoid to cause pneumonia.

Before her death, Kelly was placed into an induced coma due to organ failure and septicaemia, with her brother Murray Shaw saying in a Facebook post she had gone into cardiac arrest five times within the hour before her death.

Dr Brad McKay explained that once an infection reaches your bloodstream and you experience septicemia, commonly known as blood poisoning, you are extremely unwell.

"When you get an overwhelming infection that spills into your bloodstream... in addition to sepsis, you get all sorts of cardiovascular problems. So your blood thickens, and your blood pressure will often drop down, and once it drops down too much you can have a loss of blood flow to your heart and that's how you can have a heart attack."

Symptoms of septicemia include shivering, a fever, extreme pain, shortness of breath, sleepiness and confusion.

Meanwhile, symptoms of pneumonia include feeling more and more short of breath, a high fever, sweating, uncontrollable shaking and dizziness or lightheadedness.

Dr McKay warns if you think you may be experiencing either of these conditions to seek immediate medical attention.