The Centre for Health Protection is investigating a case of Legionnaires' disease affecting a 52-year-old man, who is in a critical condition.
He fell ill on June 19 and was admitted to Ruttonjee Hospital's intensive care unit on June 21.
He tested positive for Legionnaires' disease.
His home contacts have no symptoms.
It is the 20th case of Legionnaires' disease reported this year. There were 41 cases last year and 28 in 2013.
