More than 12,000 cases have been reported this year and at least 19 people have died (there were a then-record 9459 cases in 2004).

At one point, hospital beds were cleared of elective surgery patients to make way for dengue patients and the Government convened an international panel of experts to find out what had gone wrong.

Neighbouring Malaysia and Indonesia are also struggling to control a spike in the virus.

"Singapore is the main concern because they should not have this [problem]," says Dr Kevin Palmer, the World Health Organisation's regional adviser on dengue. "They have very sophisticated monitoring and surveillance, which does not seem to have worked."

While the world is fixated on a potential H5N1 bird flu pandemic, countries across southern Asia are dealing with a familiar fatal enemy - a dengue fever epidemic that has infected more than 110,000 people this year, from eastern India to the Philippines, killing at least 890. The rate of infection has jumped 50 per cent since last year, according to the World Health Organisation, and nobody has a good answer as to why.

"If Singapore can't control dengue, who can? It's a developed place, it's a country of tarmac and buildings designed to get rid of potential water sources. But as a model, it's not doing very well," Palmer says.

The Aedes mosquito has adapted to the constraints of its urban environment. As open-water breeding grounds have been covered, the mosquito has found abnormal breeding grounds, deep inside buildings, that are much harder to detect.

Dengue fever and the more severe dengue haemorrhagic fever are the most common mosquito-borne viral diseases in the world.

The viral infection - which has four serotypes - is transmitted by the Aedes mosquito. Exposure to one strain does not provide immunity to the other three.

It is sometimes called bone-breaker's disease because it causes severe joint pain. Other symptoms include high fever, nausea, and a rash. If it develops into dengue haemorrhagic fever it causes internal bleeding. There is no known cure or vaccine but it can be managed if treated.

The disease is spread when a female mosquito bites an infected person, mixes the virus with its saliva and passes it on when it bites the next person. It cannot be spread directly from person to person. To prevent dengue fever, you must prevent the Aedes mosquito, identified by the black and white stripes on its body, from breeding, which it does in standing water.

Singapore has used a three-pronged approach on dengue control - fogging, regularly spraying chemicals in public spaces of public housing blocks; combing, searching out and destroying breeding places and public education.

For the past six weeks, Singapore has been "carpet combing" its suburbs trying to stamp out breeding grounds. More than 9000 public housing and private properties have been checked, 920 mosquito-breeding sites found and destroyed and 300km of drains checked with 10 kilometres found to have problems.

Singapore's Environment and Water Resources Minister, Yaacob Ibrahim, told reporters at the weekend the dengue situation had eased and now it was up to Singaporeans to take care of their own homes and communities. The seven-day average of reported dengue cases has fallen to 50 (350 per week), down from 180 at the peak. Dr Duane Gubler, director of the Asia-Pacific Institute for Tropical Medicine and Infectious Diseases in Hawaii, who visited Singapore as one of the panel of experts late last month, said in some ways Singapore is a victim of its own success.

"They have the best [prevention] program in the world. The population of mosquitos is low. But because they were so effective, the [human] population's immunity is very low. With that low immunity, even a small number of mosquitoes can create an epidemic."

But the authorities also let their guard down. WHO's Palmer said recent combing had revealed previously undetected breeding places which should have been seen earlier.

"The system has been in place a long time, there is a register of the breeding places. I think what happened is the inspectors have not looked for new places."

The authorities have also assumed the infection takes place in the home. "They have not done research on where the transfer [infection] occurs. Normally you assume it is in the home but this outbreak suggests the transmission may take place in public spaces," Gubler said. If this is true, the Singaporean fogging program would have little effect.

"Singaporeans tend to think they are not at risk from these diseases, they get very upset but they have to realise they are in the tropics. No matter what they do some of these vector borne diseases are going to affect them," Palmer said.

Singapore's regional role as a transportation hub, means it will never eradicate the disease. "Dengue fever is endemic is just about every country in Asia," Gubler said. WHO lists dengue as a major public health problem in Indonesia, Burma, Sri Lanka, Thailand and East Timor.

"Many more travellers bring dengue fever to Singapore than malaria.

"You have thousands of immigrant workers coming to Singapore from dengue-endemic countries bringing the virus with them," Gubler said.

It is difficult to obtain accurate numbers on the scale of the current outbreak because reporting dengue incidence can have a serious economic impact. WHO draws its provisional numbers from media reports. Even on this evidence, Palmer said this year has been the worst in the region since 1998, when more than 1.2 million cases, with 3442 deaths, were reported.

Indonesia's dengue rates are highest, with 38,285 cases so far this year, of which 538 have been fatal, according to Indonesia's Health Ministry. Last year, almost 60,000 people were diagnosed with the virus, with 669 fatalities.

In Malaysia, the Health Ministry says that of the 31,037 suspected cases reported this year, 8889 cases were confirmed to be dengue, and 78 people have died.

Dr Ramlee Rahmat , the Health Ministry's director of the disease control division told reporters at a weekly dengue update there was reason to be optimistic as the number of cases had fallen slightly.

"We hope this positive trend will continue. We believe that beefing up monitoring and collaborating with the public in cleaning up areas has helped to keep the number of cases down," he said.

In the Philippines so far this year there have been more than 18,800 cases, a 26 per cent rise over last year, and at least 259 have died. This is seen as the tip of the problem, as case numbers are only collected from sentinel hospitals, not across the board. Thailand's dengue-fever cases have increased a little this year, to 32,193 suspected cases.

Without the threat of human to human transmission, and with little impact on developed societies, the virus is under-researched.

"People forget about dengue, it is periodic and comes in cycles, in between people get complacent," said Palmer. "Count the number of people [who died of] SARS, the number of dengue deaths every year is three times that."

From its first regional appearance in Manila, the Philippines in 1949, dengue is now endemic in most countries in the region. Different strains are prevalent in different years. The question for scientists is has the virus mutated, become more virulent.

Gubler has said "it is quite clear the disease has evolved … there is just more dengue in the world."

Despite this, there is no evidence of mutation, Palmer said.

"There is a phenomena, whereby if you get one strain of dengue this year, you will be sicker [when you get a different strain] next year. It could logically be the case but it's a theory that has not been proven."

Always remember

+Dengue is a serious viral disease transmitted by mosquito bite.

+Infection occurs in two forms: dengue fever and dengue hemorrhagic fever.

+Dengue fever is a severe flu-like illness that affects older children and adults, but rarely causes death.

+Dengue hemorrhagic fever (DHF) is more severe. Bleeding and occasionally shock occur, leading to death, mostly in children. Persons suspected of having dengue fever or DHF must see a doctor at once. Dengue hemorrhagic fever is deadly and early recognition and treatment can save lives.

+Severe abdominal pains (black stools), bleeding on the skin, sweating, cold skin are danger signs.

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