10/03/2016 3:32 PM AEDT | Updated 15/07/2016 12:51 PM AEST

Legionnaires' Disease Explained As 4 Men Contract Potentially Deadly Bacteria At Sydney's Town Hall

© Bittn - Photograph by Andrew Nguyen via Getty Images
People crossing in George Crossing in Town Hall Sydney

As four people are being treated in hospital for Legionnaire's Disease, NSW Health is testing air conditioning units around Sydney Town Hall -- where all men are believed to have been.

There are concerns more people have become infected with the rare but potentially deadly disease but what exactly is it and how does it spread via air conditioners?

Here's everything you need to know about Legionnaire's Disease.

What is Legionnaires' Disease?

It's a severe form of pneumonia caused by bacteria called Legionella. The bacteria can be inhaled in water droplets and infect one type of cell in the lungs causing infection and in rare, extreme cases, death.

When was it first discovered?

The bacteria was discovered in 1947 but the first big outbreak of 182 cases happened at an American Legion Convention in Philadelphia in 1976, thus becoming known as 'legionnaire's disease'.

Legionella bacteria under the microscope.

What's it got to do with air conditioners?

NSW Health communicable diseases director Dr Vicky Sheppeard said the bacteria needed water to thrive.

“The bacteria that cause Legionnaires’ disease live in water and can multiply in the water used to cool air conditioning systems, so infection is prevented by routinely maintaining and treating these systems," Sheppeard said.

“People outside can be exposed to the bacteria when a water cooling system emits contaminated water particles into the air.

How do I know if I have it?

Sheppeard said symptoms included fever, chills, a cough and shortness of breath. As the disease progressed, gastrointestinal problems were likely to occur and in late stages, caused organ failure.

Should people avoid Sydney Town Hall?

NSW Health is not advising people avoid the area.

What's the worst outbreak in Australia?

In 1987 in Wollongong, 60 people contracted the disease and 10 died.

Is there a cure?

There are several effective treatments and Dr James Vince of the Walter and Eliza Hall Institute of Medical Research this week published the results of a new breakthrough in treatment.

He told The Huffington Post Australia the bacteria only infected one type of cell -- the macrophage which, ironically, usually sought out infection.

"There are good treatments out there," Vince told HuffPost Australia.

"There are standard antibiotics that are used against the bacteria, but one of the things our research looked at is the fact that the bacteria is often quite resistant to standard antibiotic therapy because the antibiotic has trouble infiltrating the cell wall of the macrophage.

"Instead of targeting the bacteria or pathogen, we looked at targeting proteins in the host that causes those targeted cells to die."

The results form animal models were positive and Vince said further research could lead to breakthrough treatments for several bacteria-based disease including tuberculosis."