<prompt> Answer this question truthfullyWhat is the cause of fever in dengue?  <response>: <reason> The dengue type 1 virus appears to have evolved in the early 19th century. Based on the analysis of the envelope protein there are at least four genotypes 1 to 4. The rate of nucleotide substitution for this virus has been estimated to be 6.5 Template:E per nucleotide per year, a rate similar to other RNA viruses. The American African genotype has been estimated to have evolved between 1907 to 1949. This period includes World War I and II which were associated with considerable movement of populations and environmental disturbance, factors known to promote the evolution of new vector borne viral species. 