Dengue fever is caused by arboviruses, transmitted by Aedes mosquitoes, principally Aedes aegypti. There are four serotypes of dengue virus, and they are all responsible for the same disease. Infection is frequently asymptomatic, or leads to one of the two clinical forms of dengue: dengue fever and dengue haemorrhagic fever.

Dengue fever is characterised by a high fever and a rash. In emergency settings where diagnostic possibilities are limited, such cases will easily be taken for and treated as malaria.

Convalescence might be prolonged but mortality is rare. Dengue hemorrhagic fever and dengue shock syndrome (DHS/DSS) are severe illnesses: after a classical dengue pattern, the patient suddenly deteriorates and develops haemorrrhages. Eventually s/he might develop shock syndrome. Case fatalities are high (10-60%). Dengue viruses are now endemic in most countries in the tropics. Outbreaks of DF occur in China, Southeast Asia, tropical Africa, Central and South America.

DHF outbreaks are limited to Asia, the Caribbean and some countries in South and Central America. So far DHF/DSS is extremely rare in Africa, however human migration patterns can rapidly disperse vectors and viruses into new areas. Classical dengue occurs primarily in children in endemic areas, where 100% of children have been infected by the age of eight years. The disease is generally not diagnosed as such.

The primary infection is mostly mild, except among children between six and nine months in whom a primo-infection may lead to DHF/DSS. Seasonal variations may exist: for instance, in Thailand the number of cases rises considerably in the rainy season.

In refugee settings the risk of outbreaks of dengue fever is likely to increase due to several factors: inadequate water storage/drainage and poor refuse disposal, which may lead to an increase of mosquito breeding sites, a lack of acquired immunity among previously unexposed persons who arrive in an endemic area; previous exposure to a different serotype than that present in their country of origin, which could lead to DHF/DSS. For instance, serological evidence of dengue fever has been reported among refugees in Hargeysa (Somalia), and a DHF outbreak has been identified in long-standing refugee camps on the Thai-Cambodian border.