Millions of people who visited Sydney’s CBD over Christmas and New Years are being urged to stay alert for symptoms of legionnaires’ disease after seven people were diagnosed with the debilitating lung illness.

NSW Health issued a health alert on Wednesday for anyone who visited the city centre since Christmas Eve, or the past 10 days, to monitor for symptoms including coughing, fever, shortness of breath and headaches.

It comes after three women and four men, aged from their 20s to 70s, were admitted to hospital with pneumonia before being identified to have been carrying the legionella bacteria, which causes legionnaires’ disease.

The people are believed to have visited locations in the Sydney’s CBD between Bathurst St, Sussex St, Elizabeth St and Circular Quay, all locations frequented over the holiday period, in the 10 days prior to their symptoms.

NSW Health said symptoms of legionnaires’ disease could develop up to 10 days after the time of exposure to contaminated water particles in the air and can lead to severe chest infections such as pneumonia.

The disease cannot be spread from person to person. Instead, it is usually spread by contaminated water particles emitted by a cooling system. Smokers and those with underlying health and lung problems are most at risk.

NSW Health said environmental health officers were working closely with the City of Sydney Council to inspect cooling towers and that a review of maintenance records of could help determine further towers to be inspected.

‘Managers of buildings with cooling towers are being contacted and informed of the cluster. Building owners should ensure that their cooling towers are operated and maintained in compliance with regulation,’ they said.

More than a million people flocked to the city’s centre just on New Year’s Eve