The dengue viruses are members of the genus Flavivirus and family flaviviridae. These small (50 nm.) viruses contain single-strand RNA. The virion consists of a nucleocapsid with cubic symmetry enclosed in a lipoprotein envelope. The dengue virus genome is approximately 11,000 base pairs in length, and is composed of three structural protein genes encoding the nucleocaprid or core protein (C), a membrane-associated protein (M), an envelope protein (E), and seven nonstructural protein (NS) genes. The envelope glycoprotein is associated with viral haemagglutination and neutralization activity.

The dengue viruses form a distinct complex within the genus flavivirusbased on antigenic and biological characteristics. There are four virus serotypes which are designated as DEN-1, DEN-2, DEN-3 and DEN-4. Infection with any one serotype confers lifelong immunity to that virus serotype. Although all four serotypes are antigenically similar, they are different enough to elicit cross-protection for only a few months after infection by any one of them.

Dengue viruses of all four serotypes have been associated with epidemics of dengue fever in which there was little or no evidence of DHF. All four virus serotypes have also caused DHF epidemics associated with severe and fatal disease.