Maybe it's my inner Brooklynite talking, but to me cities are natural phenomena: Ants make hills, bees make hives, humans make cities. Each of these colonies is a meta-organism, really. They all must survive by taking in resources and then figuring out what to do with the waste that comes out the other end. The most innovative organisms and communities figure out ways to recycle waste into raw material for more housing, new appendages, or further growth.
The ability to exploit waste instead of drowning in it comes naturally to ants and bees, but it has proved tricky for us. Modern cities seem to be perpetually teetering at the brink of one crisis or another, be it chemical or microbial in origin. If the pollution doesn't get you, the influenza will. Still, such concerns pale in comparison to the survival challenges that cities used to face.
Steven Johnson makes this fact startlingly clear in his latest book, The Ghost Map: The Story of London's Most Terrifying Epidemic—and How It Changed Science, Cities, and the Modern World (Riverhead, October 2006). The book examines a preindustrial London whose 2 million denizens lacked the sort of sewage infrastructure necessary to carry all their, shall we say, effluence to a safe distance.
Remarkably, the city evolved an elaborate self-repair mechanism. An entire subculture of scavenger workers emerged to scrape, collect, and carry London's human waste to the edge of the city, where it could be used as fertilizer for nearby farms. This in turn led to greater agricultural efficiency, allowing the city to support a larger population, which led to greater waste production, and so on.
The size and population of the city organism was limited only by how effectively it could process its waste. This system worked a bit too well, and eventually London grew so large that its horse-drawn human waste carriers couldn't make it all the way to the city limits. Sewage began to accumulate, and the resulting unsanitary conditions led to a cholera outbreak in 1854.
Then the city came to its own rescue again, inspiring two men whose very collaboration might never have been possible without the urban environment. John Snow, a physician, collaborated with church leader the Reverend Henry Whitehead to develop a death density map that indirectly detected the source of the otherwise invisible outbreak: a single contaminated pump handle. Against conventional wisdom, they discovered cholera to be caused by a waterborne microbe.