As Lee Riley read article after article about the deadly 2014 Ebola outbreak, his frustration mounted. “I was seeing all of these newspaper reports and even scientific reports talking about this unprecedented epidemic in West Africa,” says Riley, a specialist in urban public health at the University of California, Berkeley, “and there wasn't a single mention of the words 'slums' or 'informal settlements'.”

Ebola is feared because of its high mortality and limited treatment options, but generally it has been limited to remote rural regions. The 2014 outbreak was different: flare-ups in cities such as Conakry in Guinea and Monrovia in Liberia revealed the havoc that this lethal virus could wreak in urban environments. The dense and highly mobile populations provided greater opportunities for the infection to spread. And according to Mosoka Fallah, an epidemiologist who was working with Liberia's Ministry of Health at the front line of the Monrovia outbreak, urban slums bore the brunt. “Wherever there were big outbreaks, most people being infected were among the poor,” he says. “Those that didn't have basic sanitation, who had the most distrust of institutions — they also had the most disease.”

Developing nations have experienced an astonishing boom in urbanization in the past few decades. The urban population of Kenya, for example, has grown at an average rate of 4.3% per year since 2010, as rural citizens have moved to cities in pursuit of new opportunities. “The range is between 3% and 6% in most of Africa,” says Robert Breiman, an infectious-disease epidemiologist at Emory Global Health Institute in Atlanta, Georgia. Riley says that there has been a similar trend in Brazil, where 85% of the population now lives in cities. Many migrants initially make their home in informal settlements at the city periphery. The United Nations Human Settlement Programme UN-Habitat estimates that 863 million people — one-third of the developing world's urbanites — live in slums.

Although better access to medical care means that the health of city dwellers across the socio-economic spectrum is generally superior to that of their rural counterparts, cities can also provide greater opportunities for infectious diseases to flourish. Crowding and poor or non-existent infrastructure exacerbate the risk of infectious disease to slum inhabitants in particular. However, as demonstrated by the rapid spread of the Zika virus over the past year, outbreaks can be a threat to entire cities, nations and — thanks to globalization — the rest of the world.

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