Dengue virus infection may be asymptomatic or may cause undifferentiated febrile illness (viral syndrome), dengue fever (DF), or dengue haemorrhagic fever (DHF) including dengue shock syndrome (DSS). Infection with one dengue serotype gives lifelong immunity to that particular serotype, but there is no cross-protection for the other serotypes. The clinical presentation depends on age, immune status of the host, and the virus strain.

1. Undifferentiated fever: Infants, children and some adults who have been infected with dengue virus for the first time (i.e. primary dengue infection) will develop a simple fever indistinguishable from other viral infections. Maculopapular rashes may accompany the fever or may appear during defervescence.
   
2. Dengue fever: Dengue fever is most common in older children and adults. It is generally an acute biphasic fever with headache, myalgias, arthralgias, rashes and leucopenia. Although DF is commonly benign, it may be an incapacitating disease with severe muscle and joint pain (break-bone fever), particularly in adults, and occasionally with unusual haemorrhage. In dengue endemic areas, DF seldom occurs among indigenous people.
   
3. Dengue haemorrhagic fever: Dengue haemorrhagic fever is most common in children less than 15 years of age, but it also occurs in adults. DHF is characterized by the acute onset of fever and associated non-specific constitutional signs and symptoms. There is a haemorrhagic diathesis and a tendency to develop fatal shock (dengue shock syndrome). Abnormal haemostasis and plasma leakage are the main patho-physiological changes, with thrombocytopenia and haemoconcentration presenting as constant findings. Although DHF occurs most commonly in children who have experienced secondary dengue infection, it has also been documented in primary infections.