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.