This has coincided with increasing numbers of epidemics of dengue fever, infecting millions of people. Countries free of dengue for decades had severe epidemics in the 1980s. For example, in Brazil, more than 500,000 dengue cases were reported in 1998 and Aedes aegypti infested all important urban centers in Brazil. As of 1999 Aedes aegypti was present in all states, and dengue transmission had been reported in 22 of 27 states. As of early 1999 all four serotypes had been documented in Mexico, Puerto Rico, Barbados, and Guatemala.
More people live in urban areas than ever before.
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.