The reason: An unfamiliar dengue virus has emerged which has the potential to spark a major outbreak.

Though officials said on Tuesday that they have managed to contain the Dengue-3 virus for now, they are still keeping a close watch on it, as it has not been seen here commonly for some 10 years.

With low immunity to Den-3 in the population, people are more vulnerable to infection.

Officials are concerned because cases increased more than three times last November and December, and were found mainly in two areas, sparking fears that transmissions were high and could spread.

There are four types of the dengue virus, and people who have been infected by any one type are immune to it for life. But that still means that an individual can get dengue fever up to four times.

In Singapore, two main types of the dengue virus have circulated in the past 10 years: Dengue-1 and Dengue-2, with the latter now predominant.

Past data shows that outbreaks in 2005 and 2007 were preceded by a switch in the predominant type of dengue virus.

In the past few months, researchers from the National Environment Agency (NEA) found that the third type, Den-3, had emerged in greater numbers in Little India and Geylang.

On average, Den-3 makes up about 5 per cent of all dengue cases here. But in November and December last year, it increased to more than 17 per cent.

Read the full story in Thursday's edition of the Straits Times.