Dengue virus is a member of the family Flaviviridae with a single-stranded positive sense RNA genome. The virus particle consists of a lipid bilayer with envelope (E) and membrane (M) proteins surrounding a spherical nucleocapsid composed of a genome RNA and capsid (C) proteins. The virion structure is easily destroyed by treatment with non-ionic detergent ( see Fig. 1 ) or low osmotic shock. Four antigenically related but distinct serotypes of dengue virus (DEN1, DEN2, DEN3, and DEN4) are circulating in tropical countries and cause dengue fever and dengue hemorrhagic fever, both of which are highly prevalent.   Fig. 1.  The effect of Nonidet P-40 treatment on dengue virus morphology. Dengue type-2 virus was purified by linear sucrose density gradient ultracentrifugation, negatively stained, and observed by electronmicroscopy. Purified virions were treated with NP-40 for 1 min.