A government-appointed panel of dengue experts formed earlier this month to study the outbreak found that the city-state's high hygiene standards had lowered residents' immunity to the mosquito-borne disease.

"It's a paradox. Because (Singapore) has been so successful (in suppressing the disease), the population that was born in the last 20 years has a very low herd immunity," panel member Paul Reiter, an entomology professor from the France-based Pasteur Institute, told reporters.

"The (Aedes) mosquito has become more effective, so you've got to go that extra mile to suppress this new level of transmission."

The city-state's dense human population has also contributed to the possibility of an epidemic breaking out, said Duana Gubler, director of the Asia-Pacific Institute for Tropical Medicine and Infectious Diseases.

"When a virus is introduced, in such a dense population that has a very low herd immunity..., you increase the possibility of secondary transmission," Gubler said.

The dengue outbreak has plunged Singapore, better known overseas for its emphasis on cleanliness, into its worst health crisis since the 2003 Severe Acute Respiratory Syndrome regional epidemic.

A spokeswoman from the National Environment Agency on Saturday confirmed that the number of cases so far this year had shot past the 11,000 mark, with the death toll at 12.

The government has set aside S$30 million (US$18 million) to fight dengue, and has been waging a "search and destroy" campaign against potential breeding areas for the past two weeks.

Unlike previous media reports attributing private residences as the main mosquito breeding grounds, the panel noted that more than half of dengue infections occur elsewhere, such as commercial buildings and public parks.

Gubler emphasised the need to implement active laboratory-based disease surveillance programmes to limit the possibility of future outbreaks.

"What we have to do in Singapore right now is to have the mosquito control operations guided by research... to find out where people are being infected," Gubler said.

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