They believe it is more and more likely that everyone who was in contact with the man - both in Taiwan and Singapore - has been identified, including during the period in which the man was travelling.

Those who have been identified are being observed. None have shown any symptoms.

A preliminary investigation, Taipei health officials reported, indicates the patient - a medical researcher - was likely exposed to the virus in laboratory setting on December 5.

The scientist then travelled to Singapore to attend a conference from December 7 to 10, and returned about noon on December 10. He began to feel sick that day and immediately knew to place himself at home in quarantine.

As many as 73 people who may have been in contact with the infected man when he was visiting Singapore have been isolated, Peter Cordingley, a Manila-based World Health Organization (WHO) official confirmed in a telephone interview.

He said the WHO had yet to receive a full report from Taiwan authorities about traced contacts there - a requirement vital for knowing for certain all encounters are investigated.

"At the moment, this appears to be an isolated incident," said Henk Bekedam, a WHO representative in Beijing.

The WHO has repeatedly asked all its member countries to ensure that SARS specimens be catalogued and their storage controlled in appropriate safety-level laboratories.

China's Ministry of Health has issued an emergency notice to ask all relevant institutes to strengthen management of SARS virus samples.

All such samples must be kept in institutes approved by the health authority.

(edited by online chief editor Li Guixiang)