Dengue fever, a usually mild albeit debilitating viral fever (breakbone
fever), is prevalent throughout the tropics, where the urban-dwelling
mosquito Aedes aegypti is a major vector. A related mosquito, Aedes
albopictus, also can act as a vector. The dengue viruses, of which four
serotypes are known (DV-1, -2, -3, and -4), are the most widespread
arthropod-borne viruses (arboviruses). They also are the only known
arboviruses that have fully adapted to the human host and lost the need
of an enzootic cycle for maintenance. During the 20th century, the
distribution and density of Aedes aegypti expanded dramatically in
tropical areas, beginning in large cities then spreading to the
countryside. This was followed by global circulation of the four DV
serotypes. Because there is no cross protection between different
serotypes, a population could experience a dengue-1 epidemic on one
year, followed by a dengue-2 epidemic on the next year. Most primary
infections cause a debilitating, but nonfatal, form of illness. Some
patients, particularly children, experience a more severe and
occasionally fatal form of the disease, called dengue haemorrhagic fever
(DHF), the most severe form of which is referred to as dengue shock
syndrome (DSS). The presence of antibodies to one serotype of DV is
believed to facilitate the occurrence of DHF/DSS in certain individuals
through immune-enhancement when infected by a second serotype. It is
estimated that from 50-100 million cases of dengue fever, 500 000 cases
of DHF/DSS and more than 20 000 deaths occur each year.