Legionella bacteria seen under microscope.

A Legionnaires' outbreak possibly linked to two Melbourne shopping centres has left two men fighting for their lives, and a third man in hospital.

The Health Department has issued a warning over the outbreak, after all three men became ill following visits to the Airport West and Gladstone Park shopping centres between April 20 and 26.

The three men, aged in their 60s and 70s, are in hospital receiving treatment.

Two are currently in intensive care.