Health officials say 88 more people from one Hong Kong flats complex have fallen ill with severe acute respiratory syndrome (SARS).
It brings the total number of cases there to 213 and has sparked fears it will keep spreading.
Two more people died today, bringing the totaldeath toll in the former British colony to 15.Officials had said the illness, which has killed 59 worldwide, seemed to spread through close contact.
Hong Kong's health secretary, Dr Yeoh Eng-kiong, announced the big rise in cases just hours after authorities imposed a tight quarantine on one block of apartment called the Amoy Gardens.
Dr Yeoh said 107 of the sick people were from the same two apartment units on several floors of the building, raising the possibility that the disease was spreading vertically.
Asked whether SARS was being spread through the air, Dr Yeoh said that could not be ruled out.
Experts believe the sickness was being transmitted mostly through droplets, when victims sneezed or coughed and nearby people were infected.
The outbreak has heightened fears in Hong Kong, where thousands of people are wearing surgical masks and much activity has ground to a halt.