Hosts can be diseased from latency condition by the decreasing of host's immunity and stimulation of condition, and so on , then become a carrier and infectious source.

外界刺激、宿主免疫力的降低等因素都可以使宿主由潜伏感染状态发病，成为带毒者和传播源，因此，对潜伏感染建立及维持机理的研究就显得尤为重要。

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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