The hemorrhagic form continues to be the leading viral hemorrhagic fever in the world. Dengue hemorrhagic fever occurs in many areas where other dengue serotypes are localized. Studies show that infection with and subsequent immunization from one dengue serotype actually increases the odds of developing dengue hemorrhagic fever during infection with a second serotype. This is especially notable in areas where multiple serotypes have overlapping, endemic regions. Dengue is predominantly found in urban areas, but can also be found in rural areas.
Epidemics are usually seasonal, during and shortly after the rainy season.
Global warming will likely expand the range of the vector mosquito. Mosquito-borne epidemics have occurred in northern Mexico. Aedes aegypti mosquitoes, which are most active during the day, are usually found near human dwellings, and are often present indoors. Elevated temperatures significantly shorten the incubation periods for the dengue virus in mosquitoes. It is currently limited to a range below 35N latitude.