More than 100 people have been infected by chikungunya in Singapore this year, a steep rise from just three to six annually for the past three years.
Most of the cases are in the Kranji-Sungei Kadut area, but a growing cluster has emerged in the Bukit Timah area.
This painful mosquito-borne disease is not endemic here, unlike dengue which is raging in the eastern part of Singapore.
In the past three years, there were between 12 and 26 cases of chikungunya a year, of which only three to six each year were contracted in Singapore.
But this year, 101 of the 107 confirmed cases were local infections. The rest got it overseas.
The cases include at least 14 people living or working in the upscale area of Fifth and Sixth Avenues, off Bukit Timah Road.
There may be more as a Health Ministry official said its officers are still investigating possible cases there.
A spokesman for the National Environment Agency (NEA) said it has inspected 543 premises in Namly Crescent and Fifth Avenue and fined 20 homes for breeding mosquitoes.
In the bigger cluster at Kranji-Sungei Kadut, however, most of the casualties are foreign workers living in dormitories.
NEA officers have inspected more than 260 factories in the largely industrial area and found mosquito breeding in 128 of them. They have been fined too.
Said the NEA spokesman: "Operations to suppress the mosquito population in the area will continue until the cluster is closed."
This will happen when there are no new cases for 15 days in a row.
Singapore had its first locally transmitted case of chikungunya in 2008. A major outbreak ensued, with 690 people coming down with it that year and another 343 the following year.
But the NEA broke the chain of transmission, resulting in only three to six locally-infected cases annually for the past three years.
Symptoms for chikungunya are almost identical to those for dengue - high fever, headache, eye ache, joint pain, rashes and lethargy. Fatigue caused by both viral diseases can last weeks or even months.
While dengue can be fatal, chikungunya rarely is, though the joint pain can sometimes last for several months.
It is also often accompanied by nausea and vomiting.
Both are spread by the Aedes mosquito, although the Aedes aegypti is more likely to spread dengue and the Aedes albopictus the chikungunya virus.
The recent hot and wet weather, as well as a change in the dominant dengue viral type, has caused a surge in dengue, leading experts to predict a huge epidemic this year.
The number of people down with dengue has been climbing every week for seven straight weeks, with 547 confirmed cases last week.
This brings the total for the year to 5,929, surpassing the 4,602 cases for all of last year.
But there have been no dengue deaths this year, unlike in the previous big epidemics.
In 2005, 14,000 people got it and 25 died; and in 2007, 8,700 were stricken with it and 24 died.