In December 2014, during the height of the Ebola virus disease (EVD) epidemic in West Africa, we wrote,“EVD is only the beginning and only one disease; even if we are to control the current epidemic, the future introduction of this and other highly contagious and virulent microbes to and from global slums is inevitable”[1]. Fast forward to late 2015: another epidemic of another virus—Zika—together with its major neurological complication—microcephaly—explodes in Brazil. Again, as with the EVD epidemic, the world’s attention is focused on issues such as where the virus originated, the need to control vectors, how quickly we can develop a vaccine, and how long the epidemic will continue. While these are important discussions to have, they are irrelevant if the world does not recognize and address a crucial reason why these explosive epidemics continue to occur in the first place: the world must talk about urban slums. Zika is, and will continue to be, a disease of the urban poor. Slum-defining characteristics—poor water and sanitation infrastructure, crowding, and poor structural quality of housing—offer ample opportunities for mosquitoes to breed and spread the Zika virus. It was recently estimated that 1.6 million childbearing women and 93 million people will be infected in the Americas’ first epidemic wave [2]. From this reservoir of infections, the world is witnessing the largest epidemic ever of a congenital complication—microcephaly. Except for sporadic reports, largely absent from discussion is the fact that the greatest proportion of Zika infections and its complications have occurred, and will continue to occur, among residents of the large, densely packed informal human settlements of Latin America and the Caribbean. The Aedes aegypti mosquito—the species that most commonly transmits Zika—will, on average, travel a mere 100 meters in its lifetime [3]. In the densely populated favelas, or slums of Brazil and elsewhere in the region, a single 100 square meter space could contain more than 100 housing structures, 2–3 stories high, with a resident population upwards of 1,000. These residents can be infected multiple times by mosquitoes circulating in such neighborhoods. Wealthy residents of high-rise apartment buildings with screened windows, air conditioning, and regular spraying of insecticides, even if located adjacent to these favelas, are less likely to be exposed to such mosquitoes.

The burden of symptomatic dengue, another mosquito-borne disease, has been shown to be greater among the neighborhoods with the lowest socioeconomic status within a slum [4]. In a study of successive dengue epidemics in Rio de Janeiro that occurred in 2007 and 2008, Rosa-Freitas et. al demonstrated that dengue incidence positively correlated with