The mosquito-borne chikungunya virus appears to have become endemic in Singapore, after it had seemingly been eradicated.
The dengue-like disease took hold in April and there have been 924 cases this year - of which 881 were locally transmitted. Last week, there were 36 new cases.
The National Environment Agency (NEA) said the bulk of the infections have been in the Sungei Kadut and Bukit Timah/Holland areas, although four of last week's cases were in the Defu Lane and Defu Avenue areas.
An NEA spokesman said: "These areas are in close proximity to lush vegetation where there is an abundance of natural habitats for the Aedes albopictus (mosquito) - the primary vector for chikungunya here."
The disease had been virtually unknown here before 2008, except for sporadic imported cases.
But in that year, a total of 690 people were infected, leading to a massive exercise by the NEA to eradicate the virus from the mosquito population.
For a while it appeared to have succeeded, with only 12 locally infected cases in the last three years.
But this year's outbreak has put paid to hopes of its demise.
Dr Indumathi Venkatachalam, an infectious disease expert at the National University Hospital, said: "We have the vector (mosquito), the virus and susceptible host (non-immune population). As long as these three factors remain, there is a risk of the disease establishing itself here."
Chikungunya is endemic in several Asean countries as well as the Indian subcontinent.
The spokesman added that the agency has carried out "multiple rounds" of fogging and misting in the chikungunya areas to try to kill the mosquitoes.
It has also taken action at mosquito breeding grounds.
According to the Ministry of Health, less than 5 per cent of chikungunya patients required hospitalisation.
The main symptoms of the virus are a sudden onset of fever and joint pain.
Often, patients also suffer from muscle pain, headache, nausea, fatigue and rash.
Chikungunya is believed to be more painful than dengue but is rarely fatal.
However, for some people the joint and muscle ache can linger on for months.
Dr Venkatachalam said her hospital has not seen many cases of chikungunya, adding that most patients are treated by general practitioners or at polyclinics.
Dengue fever is mainly spread by the Aedes aegypti mosquito.
More than 20,700 people have contracted it this year in the biggest epidemic Singapore has experienced.
