A scientist who tested positive for the SARS virus in the world's first reported infection for three months may well have caught it in the laboratory where he worked.
Dr Balaji Sadasivan, Singapore's junior health minister, said the laboratory had been temporarily shut and the man himself posed no significant health risk to others.
"The laboratory is involved on work with SARS," he said. "If further testing confirms that he did have exposure to SARS, the most likely source of the SARS virus is maybe the laboratory itself."
The man's condition put Asia on alert for a resurgence of Severe Acute Respiratory Syndrome (SARS) which spread to 30 countries earlier this year. It originated in southern China, infecting nearly 8,500 people globally and devastating regional economies.
More than 800 died, including 33 in Singapore.
Officials urged Singapore's four million residents to remain calm, stressing the low public health risk from the infected scientist, who had been isolated since Monday and was recovering quickly.