Poor housing and absence of screens increases the potential contact between mosquitos and humans. It is estimated that a population size between 150,000 and 1 million is needed to sustain the circulation of a dengue virus and to increase the risk of severe forms of dengue fever. Recent studies also point to another reason to be concerned about population size. Over the past two centuries the number of dengue lineages has been increasing roughly in parallel with the size of the human population. Scientists have suggested that a larger human population in which the virus replicates allows increased opportunities for viral evolution and enhances the potential for the appearance of more virulent strains.
Global travel continues to increase.
This massive movement of the human population links all major urban areas of the world. Speed of travel means that a person bitten by a mosquito carrying dengue in Bangkok can reach home in Brazil before symptoms of infection begin. Travelers carrying the dengue virus have transported it around the globe and introduced different serotypes into new areas where competent vectors exist. It is the co-circulation of more than one dengue serotype that sets the stage for severe disease. When dengue epidemics occur in Latin America, the virus also infects travelers to the region.