The World Health Organisation (WHO) has issued a global pneumonia alert after a highly contagious respiratory illness killed one man and infected nearly 60 hospital staff in Hong Kong and Vietnam.

Health authorities have not been able to establish any link between outbreaks in the two places, but experts studying the Hong Kong cases believe they were caused by rapidly spreading viruses that have yet to be identified.

"It must have come from a patient and is probably spread from human to human in very close contact by aerosol, like sneezing," said Professor John Tam, who is among a team of doctors treating those battling the virus in Hong Kong.

"It affects a lot of people very quickly and their conditions can deteriorate very quickly," Tam said.

An American businessman, who died on Thursday in a Hong Kong hospital, had been flown in from Hanoi, where he had been admitted to hospital shortly after arriving from Shanghai and Hong Kong with severe respiratory problems, the WHO said.

Following his admission, some 20 staff in the Hanoi hospital fell sick with similar symptoms and some have developed pneumonia and acute respiratory distress, it said.

Some of the Hanoi victims remain critically ill.

That hospital has since been shut for cleaning and will not reopen until late next week, according to an employee there.

In Hong Kong, more medical staff have fallen ill with flu-like symptoms. By late Thursday, 38 hospital staff had been admitted and 24 of them were showing signs of pneumonia.

None of those in Hong Kong is known to have had any contact with the American. Two of the 24 are in serious condition.

FAST, DEADLY

Vietnam's state-run media on Thursday quoted Vice Health Minister Nguyen Van Thuong as saying: "Preliminary results showed this is the influenza virus type B."

Earlier this week an official at the Hanoi-based Epidemic Hygiene Institute told Reuters: "The virus can be fatally dangerous to humans and it can spread really fast."

She added: "There is no specific vaccine for the virus as it keeps changing from year to year."

Hong Kong's health minister Yeoh Eng-kiong told reporters: "There is a possibility that it can mutate or it may be a virus that is not known to us."

News of the infections worried many in Hong Kong. In some of the territory's hospitals, staff and patients, and even those seeking outpatient treatment, wore surgical masks.

Pneumonia is the number four killer in Hong Kong where some 300 people are admitted into hospital every week.

But the current cases in Hong Kong are of special concern, coming so soon after a much-feared bird-flu virus killed a man and infected his son in February. The father's flu-like illness deteriorated into pneumonia before he died.

A government spokesman said earlier that the latest virus was not H5N1, or the deadly bird-flu virus which sparked a global WHO alert and widespread panic in February.

Six people in Hong Kong died in 1997 of the avian virus, which jumped mysteriously from bird to human.

There was an outbreak of atypical pneumonia in China's southern Guangdong province in mid-February which infected 305 people, killing five. It is not known if the Guangdong cases are linked to the ones in Hanoi or Hong Kong.

Editor: Weiwei