So far Queensland has been the state hit hardest, but health authorities say the worst is yet to come across the country.

So how is it spreading, who's most at risk, and are there any ways to avoid being struck down?

It’s flu season and we're in the eye of the storm.

More stories from Today Tonight

• Whooping cough epidemic
• Monster health insurance premiums
• Killer bug health warning

Grieving parents Helen and Noel Delahoy know how lethal the virus can be. It took just a few weeks for their fit son Scott to succumb to H1N1 - better known as Swine Flu.

Scott was 44 and had been in the United Kingdom with his wife Phillipa. He thought it was just a bad cold, but 24 hours after landing back in Sydney he was on life support.

The virus was ravaging his body and Scott lost his fight.

“Every day you have something that triggers you to think about Scott and you dream about him a lot at night,” Helen said.

Scott had skipped having his annual flu injection. “I believe everyone should get the vaccine - it's too late once you're dead,” said Noel.

Dr Allan Hanson knows two of his old flu foes are back in town.

Today Tonight's latest health stories

“I've been working with flu one way or another since mid to late 1960s - working on making vaccines, studying the effects of influenza virus,” Dr Hanson said.

“The two strains that are circulating this year in the population are Type B Influenza Virus and the Type A H3N2 Influenza Virus. They really haven't been prominent in our population for the last three years and so population immunity is probably lower, and that will probably mean more cases and potentially more severe cases.”

H3N2 is also spreading, and this relatively new strain (in flu terms) targets mostly those over 65, while Swine Flu hits younger adults, and Type B mostly singles out our children.

More stories from reporter Clare Brady

“Type B generally affects children more severely than anybody else, and so childcare, schools and the like, these are places where we will see potentially quite serious outbreaks,” Dr Hanson said.

A simple sneeze can be a potential killer. It contains hundreds of viruses and can deliver the flu. The virus lands on a throat cell and then locks on a receptor, which welcomes it in. The virus is then pulled down into the cell, bursts, and the virus is away, eventually breeding millions of replicas.

The immune system is then sent into battle - but sometimes loses against the flu.

In an odd twist, it's often our immune system that makes us feel dreadful.

“The body responds by putting out various chemicals that are supposed to stop the virus, but those chemicals are then the things that then make you feel ill. They give you the fever, they give you the aches and pains, you don't actually have the virus in the muscles of your body, you have the result of the chemicals that the virus has spread through the body,” Dr Hanson explained.

This year there have been more than 9000 confirmed cases of the flu, with Queensland, New South Wales and South Australia fairing the worst, and children under the age of nine struggling the most.

“With 92 per cent of all absenteeism caused by sick leave, it's actually costing the economy around $25 billion a year. So it's a significant cost,” said Paul Dundon of Direct Health Solutions.

Dundon is keeping track of this costly virus. “For businesses alone, it's costing $3500 per person per year,” he said.

So how can you prevent the spread of flu?

• Washing hands;
• Using a medicated hygiene wash;
• Covering your mouth when coughing;
• When you have the flu isolate yourself;
• Plenty of rest.

“You really do need to rest, it’s for your own benefit, and for the benefit of the people around you so you're not infecting them,” Dr Hanson advised.