Legionnaires' disease a type of pneumonia caused by exposure to legionella bacteria, it is life-threatening but rare.

Symptoms include fever, chills, a cough, shortness of breath and aching muscles.

Associate Professor and specialist in Infectious Diseases The Australian National University Sanjaya Senanayake said gardeners should wear a face mask and gloves while handling potting mix.

"Legionella outbreaks are typically associated with contaminated water cooling towers. But it is a bacterium that is also found in the soil," he said.

"One species in particular, Legionella longbeachae, can be inhaled from potting mix. That is why it is recommended that people using potting mix should wear a mask and wash their hands afterwards."

In May this year, five people were admitted to hospital after contracting Legionnaires' disease after visiting Sydney's CBD.

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Those at greatest risk of Legionnaire's disease are the elderly, smokers and the immunocompromised.

People who develop Legionnaires' disease are diagnosed by chest x-ray and a urine test and usually require antibiotic treatment in hospital.

"Most people using potting mix won't get Legionella pneumonia, but antibiotic treatment is available for those who do," Dr Senanayake said.

"The infection is named 'Legionella' because it was first isolated in relation to an outbreak at the American Legion's Convention in Philadelphia."