The Centre for Health Protection is investigating a case of Legionnaires' disease affecting an 87-year-old man who lives in Zhuhai, Guangdong.
He fell ill on August 25. He came to Hong Kong on August 27 and was admitted to Queen Elizabeth Hospital.
The clinical diagnosis is pneumonia and he is in a serious condition.
He tested positive for Legionella pneumophila.
The patient had travelled to Hong Kong from August 19 to 21 during the incubation period and stayed on the Mainland before falling ill.
His home contact has no symptoms.