Dengue cases rose sharply at the end of last year. The week of Dec 27 to Jan 2 saw 455 cases - the highest for the year and 86 more than in the previous week.
It brings the total number of people infected last year to more than 11,000, with four dying from this mosquito-borne viral disease.
The victims are mainly from the eastern half of the country, with the largest of the 97 active clusters in Tampines, which has 178 cases.
Dengue had been quiet for most of the year, with the number of infections below 300 cases a week.
But December, which is not usually the high dengue season, had three weeks of more than 300 cases and one with 455, possibly signalling a big outbreak.
The last time that dengue numbers topped 400 in a week at the turn of the year was in 2013 - the year of the worst dengue outbreak, with more than 21,000 people diagnosed with the disease.
The National Environment Agency (NEA) warns that it has been seeing a rise in the number of aedes mosquitoes - the ones that transmit the dengue virus - likely caused by the warmer year-end weather, leading to mosquitoes breeding and maturing faster.
Another worry is the change in the dominant virus. Den-1 has been responsible for most of the dengue cases here since March 2013, said the NEA. But the Den-2 virus now accounts for more than half of the cases. Going by past experience, each time the dominant strain changes, the result is almost always a surge in cases.
Studies show that the number of diagnosed cases is the tip of the iceberg, with real numbers more than 20 times the official figures.
This is because most people would see a doctor only if they are very sick. Symptoms include sudden high fever, joint and muscle pain, vomiting and a red rash.