Growing numbers of rural poor are migrating to cities around the developing world, giving aid experts a new cause for anxiety - beyond just the deplorable conditions of the burgeoning slums.
The concern goes like this: living in such close quarters with the urban rich exacerbates the disparities between the "haves" and the "have-nots," fueling a global explosion in crime, street violence, and extremism. The urbanization of poverty, say development experts, could become as much a world-security issue as the hunt for Osama bin Laden or weapons of mass destruction.
"Relative deprivation has always been a flash point," says William Masters, interim executive director at Colombia University's Center for Globalization and Sustainable Development in New York. "Globalization has opened up comparisons and created expectations that make the gaps in world living standards increasingly unacceptable."
To try to close these gaps, a handful of projects around the world, including an ambitious one here in Egypt's capital, are bringing hope to some of the most squalid places on earth.
The numbers worldwide are staggering: according to a new report by the United Nations' Habitat, the number of global urban poor has now crossed the 1 billion mark, and at current rates of growth that number will double in three decades.
Slum dwellers already make up almost one-third of the world's urban population, residing mostly in the developing world, where few governments have the financial resources to cope. Cairo, for example, gets 1,000 new residents every week, even though jobs are scarce and housing supplies badly strained.
Conditions in most developing world slums are beyond deplorable: hundreds of thousands of residents crammed into small areas without sewage and running water, living over stinking garbage dumps, drinking from polluted water sources.
Here in Cairo, a vast population of squatters has moved into an ancient tomb city, turning caskets into tables and chairs in a community where the living have taken over the homes of the dead. The 300-page UN report cites a critical shortage of funding from international donors to make urban slums in places like Karachi, Pakistan; Sao Paolo, Brazil; and Jakarta, Indonesia more livable.
Though the cost of dragging 1 billion people, or even 10 percent of them, into relative prosperity may seem daunting - the report puts the figure in the billions - it also suggests that community-driven projects, which aim to revitalize poor areas by helping residents develop new trades, can improve economic conditions immediately and become self-sustaining.
And while studies show that slum dwellers are more often the victims of crime and violence than the perpetrators of it, there is equal evidence that all too many urban poor, out of desperation or simply frustration, turn to transgression or extremism. Drug and crime gangs from the slums of Bogotá, Colombia; and Mexico City have been linked to some of the world's most powerful trafficking groups. Suicide bombers who took part in the deadly May terrorist attacks in Casablanca, Morocco, came from crowded shantytowns around the city. Poverty and lack of opportunity across the Muslim world is widely blamed for the rise in Islamic extremism.
"There needs to be a far greater sense of urgency about this," says Professor Masters.
Cairo's Darb al-Ahmar district, its narrow alleys lined with teetering shacks, strewn with rotting garbage, and prowled by ruthless drug dealers, was until recently one of this city's most ghastly and disreputable slums.
Now the ancient neighborhood, home to 250,000 people, is a hive of activity. Community members wearing hard hats toil at construction sites, rebuilding their own homes and erecting a school, a landscaped park, and a community center.
"It's nice to finally have a steady job," says Sayeed Rushdi, as he surveys the rubble-strewn construction site that used to be his neighborhood. "And later, when my home is fixed, we will really feel how much this has helped us."
Neighborhood workshops that make ceramic tiles, carve wooden moldings, or forge metal door frames are doing a booming business, supplying materials the community needs to rebuild. The workmen have been trained in their various crafts by experts, and their tiny shops now boast modern machinery.
There are microcredit programs, vocational training classes, and even a new local choir. It's all part of a $45 million, seven-year project by the Aga Khan Development Network, aiming to breathe vitality into an urban slum and give residents there the tools they need for a better life.
"This project has allowed us to save the urban fabric of this historic neighborhood," says Mohamed el-Mikawi, the project manager. "More important, we are creating jobs and giving them a future."
It's the kind of holistic, costly, and complicated project experts say the world needs many more of.
Other community-driven projects have made a difference in small pockets around the globe: housing-development programs have produced hundreds of homes and jobs in Karachi, Pakistan; and Bangkok, Thailand. And a clean-toilets initiative in the slums of Bombay has created 500 maintenance jobs and provided residents a safer form of sanitation.
But more often, experts say, projects fall short - either they're too small in scope and funding to affect more than a handful of families, or government efforts to stamp out illegal squatter communities block real help from reaching the people who need it most.
Even the successful ones, like in the Darb al-Ahmar case, are terribly complex to coordinate - and to keep everyone happy.
"At first, no one wanted to contribute cash to the refurbishment of their homes," says architect Ayman Elgohary, of a plan where Darb al-Ahmar residents pay 20 to 80 percent of the cost to rebuild their residences. "Now that they see the results, everyone is fighting to get on the priority list."
And change can be jarring for some, especially when there are costs involved. "We had to pay 1700 pounds ($274) to fix this house, and look," cries Najib Attia, pointing at a tile on his new floor. "That one has a crack!"
His wife snorts, noting that the previous structure was about to collapse entirely, had no windows, running water or electricity. "Now we have windows and lights," she says. "It's much better than that old place."