Symptoms vary according to age. Infection with dengue confers immunity to infection with the same dengue serotype, but aside from short-lived protection does not prevent infection with other serotypes. The condition of some patients progresses to shock. While the mechanisms that cause DHF are not completely understood, it is widely accepted that antibodies from previous dengue infections can predispose some individuals to develop DHF when infected by a second dengue serotype. Thus the co-circulation of several different dengue serotypes in a geographical area favours the occurrence of DHF in that area.
Mosquitos acquire the virus while feeding on the blood of an infected person.
Once infected, a mosquito is capable of transmitting the virus to susceptible individuals for the rest of its life, during probing and blood feeding. Infected female mosquitos may also transmit the virus to the next generation of mosquitos by transovarial transmission i.e. via its eggs, but the role of this in sustaining transmission of virus to humans has not yet been delineated. Humans are the main amplifying host of the virus, although studies have shown that in some parts of the world monkeys may become infected and perhaps serve as a source of virus for uninfected mosquitos. The beginning of these more frequent reports coincides with the time that the Ae. The mosquito adapted very well to urban environments, living in close proximity to people, breeding in small containers that collect rainwater and in water storage vessels.