Demographic changes occurring in underdeveloped countries due to intense rural-urban migration since the 1960s have resulted in overcrowded cities with multiple deficiencies, particularly in housing and basic sanitation. Some 20% of the population in large and medium-sized cities live in slums or under similar conditions. Lack of regular water supply and public garbage collection foster the proliferation of potential breeding sites for Aedes aegypti (the main mosquito vector for dengue), including precarious reservoirs for potable water and disposable recipients which accumulate water, like used cans and plastic and glass bottles. Modern industries also produce large volumes of disposable materials. Propagation of the dengue virus and the spread of dengue vectors are favored by the high intensity, frequency, and speed of private and public transportation. Such factors can help explain the re-emergence of dengue, the most important arbovirus in the world today, affecting thousands of people each year.