The milder dengue is similar to classic dengue, except that it lasts less than 72 hours. The reasons for the different forms of dengue is an enigma. In general, few deaths were attributed to dengue until the 1950s. Since this time a tremendous increase in DHF, relative to classic dengue, has occurred in Asia and elsewhere. The reasons for the increase in this form is largely unknown.
Hypotheses deal with the ecological and immunopathological ramifications of the disease.
Some researchers suggests that a more virulent form of the disease developed when A. albopictus somehow transferred the disease to A. aegypti . This might have occurred if Malaysian monkeys infected with a form of dengue were fed upon by A. aegypti . This is not a very well supported hypothesis, since outbreaks of DHF have occurred where monkeys are absent. A more accepted hypothesis states that DHF is caused by sequential exposures to different serotypes. Primary infection with type 2 only causes classic symptoms.