Health officials have issued a global warning about a virulent flu and pneumonia that has swept through hospitals in Hong Kong and Vietnam.
One man has died, dozens of hospital workers have fallen ill, and some of these are reportedly in a critical condition.
The precise origin of the infection is still unknown, but the key case involved an American businessman who died on Thursday morning in a Hong Kong hospital.
He had arrived into Hanoi in Vietnam from Shanghai on a business trip already suffering from severe respiratory problems.
When his condition deteriorated, he was transferred to Hong Kong for specialist care.
 It affects a lot of people very quickly and their conditions can deteriorate very quickly  Dr John Tam, Hong Kong
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It has issued a "global alert" warning public health surveillance teams throughout the world to be vigilant for outbreaks.
"Any time you cannot diagnose the cause of a disease, it's very serious," said the World Health Organisation's David Heyman.
"What's fortunate is that at present it seems to be confined to hospital workers who have had contact with the patients; although now we're looking to see if people might be sick in the community."
Rapid spread
After the US patient's admission in Hanoi, 20 hospital staff fell ill with the same infection, and so far 50 staff in Hong Kong have shown similar flu-like symptoms and some have been admitted to hospital.
Of these, 23 are already showing clear signs of pneumonia. The condition of some of these is described as "critical".
None of the staff are known to have had any direct contact with the American - so no direct link with either his death, or the Hanoi outbreak, can be established.
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Singapore's Ministry of Health says it is also closely monitoring three people admitted to hospital with an unusual form of pneumonia, all of whom had recently visited Hong Kong.
Professor John Tam, one of those treating the virus patients in Hong Kong, said: "It must have come from a patient and is probably spread from human to human in very close contact by aerosol, like sneezing.
"It affects a lot of people very quickly and their conditions can deteriorate very quickly," Mr Tam said.
Bird-flu ruled out
Concerns over the highly-contagious illness have prompted many staff and patients attending hospitals in Hong Kong to wear surgical masks.
The signs and symptoms of the disease in Hanoi include initial flu-like illness, with rapid onset of high fever followed by muscle aches, headache and sore throat.
In some, but not all cases, this is followed by double pneumonia - with some patients needing help from a ventilator to stay alive.
Hong Kong Health Minister Yeoh Eng-kiong said that testing was being carried out to try to identify the strain of virus involved.
Tests have so far revealed that it is a "B" strain version of flu.
He said: "The rate of infection is very high.
"At first it's a respiratory illness and it deteriorates into pneumonia.
"There is a possibility it can mutate or it may be a virus that is known to us."
But Hong Kong Government officials said the illness was not related to a strain of "bird-flu" that killed a man in February.
It is possible the outbreak is linked to a spate of "atypical pneumonia" cases in the southern Guangdong province of China in February, which killed five people and infected hundreds more.