Dengue is transmitted by several species of mosquito within the Aedes genus, principally A. aegypti. The virus exists in four different types; infection with one type usually gives lifelong immunity to that type, but only short-term immunity to the others. Subsequent infection with a different type is believed to increase the risk of severe complications. As there is no vaccine, prevention is sought by reducing the habitat and the number of mosquitoes and limiting exposure to bites.
People infected with dengue virus are commonly asymptomatic or only have mild symptoms such as an uncomplicated fever.Others have more severe illness, and in a small proportion it is life-threatening.The incubation period ranges from 3–14 days, but most often it is 4–7 days.
The febrile phase involves high fevers, frequently over 40 °C (104 °F) and is associated with generalized pain and a headache; this usually lasts two to seven days.Flushed skin and some small red spots called petechiae, which are caused by broken capillaries, may occur at this point,as may some mild bleeding from mucous membranes of the mouth and nose.
The critical phase, if it occurs, follows the resolution of the high fever and typically lasts one to two days.During this phase there may be significant fluid accumulation in the chest and abdominal cavity due to increased capillary permeability and leakage. This leads to depletion of fluid from the circulation and decreased blood supply to vital organs.During this phase, organ dysfunction and severe bleeding may occur.Shock and hemorrhage occur in less than 5% of all cases of dengue.