The Dengue virus is a member of the virus family Flaviviridae and is transmitted to people through the bite of the mosquitos Aedes aegypti and Aedes albopictus. Dengue virus is believed to be the most common arthropod-borne disease in the world.
It is important to understand why an individual will develop DHF/DSS. The Dengue virus has been shown to have 4 subtypes, which are different strains of dengue virus (designated DEN1, DEN2, DEN3 and DEN4) that have 60-80% genetic similarity between each other. There are subtle differences in the surface proteins of the different dengue subtypes. After a person is infected with dengue, immune response is mounted against that dengue subtype, producing specific antibodies to that subtypes surface proteins. However, if another subtype of dengue virus infects the individual, the virus will activate the immune system to attack it as if it was the first subtype. The immune system is tricked because the 4 subtypes have very similar surface antigens. The antibodies bind to the surface proteins but do not inactivate the virus. The immune response attracts numerous macrophages, which the virus proceeds to infect because it has not been inactivated. This situation is referred to as Antibody-Dependent Enhancement (ADE) of a viral infection. This makes the viral infection much more acute.
So, previous infection by one subtype provides life-long immunity to that subtype, but predisposes to a more acute form of Dengue by the other subtypes.