Immunity to dengue is low in Singapore's population because of the intensive dengue-carrier controls, "making the population vulnerable to outbreaks, " said Ng Lee Ching, director of the Environmental Health Institute at Singapore's National Environment Agency.

WSJ: Dengue Fever on the Rise in Singapore

Some factors behind this year's outbreak include a higher mosquito population and "fitter viruses with higher epidemic potential, " said according to Ng Lee Ching, director of the Environmental Health Institute at Singapore's National Environment Agency.

WSJ: Dengue Fever on the Rise in Singapore