Concern is growing about the illness across east Asia
The World Health Organisation has taken the rare step of issuing an emergency travel advisory amid fears that a mystery virus which has infected scores of people in Asia may be spreading.
The WHO has so far not advised travellers to avoid any particular destination, but has warned them to watch out for symptoms, including a high fever, difficulty in breathing, and coughing.
In the latest development, a doctor from Singapore was taken off an airplane on Saturday and quarantined in a Frankfurt hospital, German health authorities say.
Two people travelling with him on the flight from New York to Singapore were also put in quarantine.
The WHO says it has received reports of more than 150 suspected new cases of Severe Acute Respiratory Syndrome (SARS) during the past week.
AFP news agency reports that 40 people have been infected in Vietnam.
It is thought the Hanoi outbreak started last month after an American businessman travelling from Shanghai infected hospital workers; he died in Hong Kong.
In Canada, a mother and child have reportedly died from the flu, while officials in Singapore report 16 cases.
In Hong Kong, 47 medical workers are thought to have the virus; the Taipei authorities have reported three cases.
The flu is thought to be highly contagious
No figures are available yet from China where it is thought there are many cases.
"This syndrome, SARS, is now a worldwide health threat," said WHO Director-General Gro Harlem Brundtland.
"The world needs to work together to find its cause, cure the sick and stop its spread."
The man quarantined in Frankfurt had apparently already exhibited symptoms of the mystery pneumonia while in New York.
While he and two others were taken to hospital, other passengers who got off in Frankfurt were sent home and told to stay there.
People continuing their journey to Singapore will be met by health officials there.
Singapore and Taiwan have warned their citizens against travelling to the worst affected places - Hong Kong, China and Vietnam.
The Thai authorities have also imposed strict procedures to try to guard against the illness, instructing airlines to report immediately if any passengers display symptoms.
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.