A Vietnamese nurse has become the latest person to die after contracting a mystery virus, bringing the worldwide death total to 22.
Doctors are hunting the cause of the illness
The illness, known as Severe Acute Respiratory Syndrome or SARS, had already killed another Vietnamese nurse and a French doctor working at the Vietnam-France Hospital in Hanoi.
All three are thought to have been exposed to the virus after treating an American businessman admitted with a flu-like illness in February, who later died in hospital in Hong Kong.
A male health worker from the Hanoi hospital is under observation in Tourcoing Hospital in north-west France, suspected of having SARS.
In a separate move, more than 700 people in Singapore are to be quarantined in a bid to contain the spread of the illness.
We might well find that the disease, or the agent that causes that disease, has been around for a long time
Dr John Mackenzie, World Health Organization
Sixty-five people, including three children, have been infected in Singapore. Twelve are in a serious condition,
including eight who are on a ventilator.
News of the latest cases comes as a World Health Organization (WHO) expert claims the condition could have existed, unnoticed, for many years.
Dr John Mackenzie, an Australian virologist, told AFP news agency: "We might well find that the disease, or the agent that causes that disease, has been around for a long time, but we haven't recognised it previously.
"You don't know it's a new disease, really, until this cluster of cases occurs as an epidemic."
Hundreds of people are thought to have been affected by SARS across the world.
The death toll includes five cases in the Chinese province of Guangdong, thought to have been the result of SARS.
The head of Hong Kong's hospitals, Doctor William Ho, has been taken to hospital with symptoms of pneumonia.
A spokeswoman said it was not yet known if Dr Ho was suffering from the illness.
The WHO has issued a global alert, the region's tourist industry has suffered and sports events have been cancelled because international players have refused to attend.
For the people of Hong Kong, the latest news about Dr Ho - whether or not it proves to be the syndrome - will heighten fears that the virus is spreading more rapidly than previously thought.
Four schools have been closed in Hong Kong and nearly 200 children have been told to stay at home because members of their families are known to be affected.
Teams from the WHO have been despatched to China, because the illness is thought to have originated there.
But as yet, there is no cure. And although scientists in Hong Kong have isolated the virus that causes the illness, they are still a long way from developing a vaccine.