There are four main types of closely related viruses that cause dengue. A person infected with one virus type, however, does not become immune to the other three. To the contrary, with every new infection, the disease actually gets worse. Repeated infections increase the risk of the disease progressing to dengue haemorrhagic fever (DHF), a potentially fatal complication.
 

The viruses are kept in a cycle that involves humans and Aedes aegypti, a domestic, day-biting mosquito that prefers to feed on humans.