If it happens, the measures taken will include quarantine for those in close contact with the patient, as was done during the 2003 Sars crisis. But there is no need for more drastic steps taken then, such as closing the patient's workplace.

This is because the H1N1 virus, while contagious, has so far been mild, with patients who are treated early recovering quickly.

A place would be shut down only if the patient had been cloistered with many others, raising the chances of more falling ill. Such places would include childcare centres and foreign worker dormitories.

Describing what will be done if a student, worker or traveller is diagnosed with the bug, Health Minister Khaw Boon Wan said yesterday: 'We must know how to react to the first few cases. We need Singaporeans to understand our rationale so that there is no misunderstanding or panic when cases do emerge.'

He also cautioned against being fixated on the number of confirmed cases or where else the virus has spread to.

'The public health focus should be on the severity, if any, of the new cases: Do the patients develop complications, pneumonia, severe respiratory failure? Who are those vulnerable to developing complications so that we can focus on treating them early?'

The virus first appeared in Mexico last month and has spread to at least 30 countries, landing in China on Monday and in Thailand yesterday. Most cases have been mild. The United States has eclipsed Mexico as the country with the most cases, but only three people have died there.

The low fatality rate in the US is the reason why there is no move yet to quarantine those arriving from North America, said Mr Khaw.

Likewise, given the mild nature of the virus infecting those who had travelled to Mexico, the Health Ministry is also reviewing the current policy of quarantining those who had been there recently.