Sydney's central business district is under a health alert following the confirmation of a fourth case of Legionnaires' disease. Authorities are urging individuals who were in the Clarence Street area between December 9 and 18 to monitor for symptoms. Investigations are ongoing to pinpoint the source, with cooling towers being inspected. The disease, a severe pneumonia caused by Legionella bacteria, does not spread person-to-person and symptoms can appear two to 10 days after exposure.