THE dominant dengue virus strain causing the current epidemic is a "fit and healthy" one which can replicate inside mosquitoes at three times the speed of strains that circulated last year.
Dr Ng Lee Ching, director of the Environment Health Institute, explained that the insects are becoming infectious much faster after getting the virus from a human with the disease, therefore this virus strain carries a higher epidemic potential.
The National Environment Agency (NEA) said it does not know if the virus replicates itself as rapidly inside humans, as no tests have been done on this.
The Den-1 virus now accounts for more than half the infections in Singapore - and this particular strain for 40 per cent of all infections.
More than 9,600 people have been diagnosed with dengue this year, with 820 falling ill last week.
The strain of Den-1 circulating today is different from the one that caused the 2005 epidemic - until now the worst that Singapore has experienced, with 14,000 people being infected and 25 dying of dengue that year.
New, stronger strains of Den-2 and Den-3 have also appeared since the end of last year.
Coupled with that, the decades-long fight to stamp out mosquitoes means most people here have no immunity against the disease.
Among people aged 16 to 30, fewer than one in six are immune to the Den-1 virus, said the NEA.
People who have been infected are only immune to that dengue type, but not to the others.
Dr Ng said the dense population also makes it easier for the disease to spread, as seen in the "protracted transmission" resulting in large infection clusters of more than 100 people.
The largest of 59 existing clusters is one comprising 166 people in Tampines.
Dr Ng said that while dengue infections are centred mainly in the east, there is a high chance of the disease moving to other parts - as is already beginning to happen.
In 2000, the Aedes mosquito which transmits the disease was found almost entirely in the east, with some in a small area in the north.
NEA surveillance over the years has discovered that the mosquito is now found across the island, with large numbers present in places such as Bukit Batok, Clementi and Pioneer North - putting residents there at risk in the current epidemic.
Minister for the Environment and Water Resources Vivian Balakrishnan said: "Some of these high-risk areas haven't exploded yet, hence my concern."
Dr Ng said another reason for the high numbers could be that the public and doctors are more aware of dengue today, which could have resulted in more cases being reported.