Dengue infection caused by dengue virus (DENV), a flavi-virus with four distinct serotypes, DENV- 1, 2, 3 and 4, is a large health concern worldwide. An estimated 2,5 billion people live in areas where they are at risk for infection. Each year there are 50-100 million new cases of dengue infection, among these 500 000 contract the serious forms, dengue haemorrhagic fever (DHF) and dengue shock syndrom (DSS).DENV is mainly transmitted by the female Aedes aegypti mosquito. Disease symptoms include fever, headache, myalgia, arthralgia, increased tendency for skin bleeding and internal bleeding. Infection causes hematological changes like thrombocytopenia and hemoconcentration. Treatment is symptomatic, there is no vaccine currently available, but this is an important field of research.Several factors in the host and in the virus influence the severity of the disease. We have studied eight of these factors using several medical databases. We found that secondary infection increases the risk for DHF because antibodies from a previous infection may enhance the entry of a second DENV into macrophages leading to more serious infection. Some serotypes are more virulent than others, with DENV-2 as the most likely to cause serious illness. The different serotypes can be divided into genotype groups with different virulence.Of the host the factors that seem to increase the risk for DHF/DSS include; high BMI, Caucasian or Asian ancestry, age under 16 (especially between 5 and 9) and blood type AB. We found no significant difference in risk between males and females.