Mr Khaw Boon Wan said he had been keeping his fingers crossed that the mosquito-borne disease will not surface here.

But it was 'a matter of time' it arrived, given that it had taken root in the neighbouring countries, he said.

Thousands of people in Sumatra, Indonesia were infected with the virus last month, while Malaysia was hit with an outbreak in Perak two years ago. Thirty villagers were infected then.

Patient No. 11 is a Singaporean who lives in MacPherson but works in Little India.

Speculation is that he caught the disease while waiting for a bus in Little India, outside of the Clive Street cluster, which was where the first 10 patients lived and worked, indicating that the transmission area could have spread.

Environment and health officers are combing the area within 150 m radius of the bus stop, and taking blood samples from those living there.

Close to 1,800 people have been screened for the virus, since the first case surfaced in Clive Street two weeks ago.

To combat transmission, patients who still have the virus circulating in their blood have been isolated in the Communicable Disease Centre (CDC).

Read the full story in tomorrow's edition of The Sunday Times.