New plagues, survival, and the mutual adaptations carried on with our microbial fellow travelers

The glory and squalor of cities

From Babylon to Paris, the metropolis has evoked images of splendor and apocalypse. Thomas Wolfe, the prototypical small-town boy infatuated with big cities, called them "the places where we feel our lives will be gloriously fulfilled, our hungers fed."

Cities promise new pleasures, risks, friends, lovers, fortunes. They are the electrical poetry of group life.

Cities, however, have also inspired a dark poetry of decay and misfortune.

From their beginnings until the twentieth century, cities have been pestholes. In fact, only when towns became big cities did massive die-offs become a regular part of human life.

When farmers and villagers started crowding into cities, this immunologically virgin mass offered a feast to germs lurking in domesticated animals, wastes, filth, and scavengers.

Countless people were sickened and killed by previously unknown epidemics: smallpox, measles, mumps, influenza, scarlet fever, typhus, bubonic plague, syphilis, gonorrhea, and the common cold.

Many of these diseases attacked with a savagery they rarely show today, demoralizing entire societies. If we are to see why new epidemics are again striking an increasingly urbanized world, we must understand why plagues and cities have always developed together.

Permanent farms and villages made death by disease far more frequent. Urban masses became sufficiently large and dense to support zymotics, or "crowd diseases", what in other species are called "herd diseases".

The reason epidemics did not take hold until urban times is simply the conditions imposed by numbers. While nomads were not free of infections, their most common diseases were chronic, not acute.

Deadly epidemics remained relatively limited and infrequent for the same reasons they had been so among hunter-gatherers. People did not live densely packed together, which facilitated the transmission of germs from one person to another. Their settlements were sufficiently far apart, and travel was sufficiently limited, to keep outbreaks of disease localized.

In addition, most bacterial and viral infections with epidemic potential left survivors temporarily or permanently immune. When such diseases did jump from an animal source to nomads or villagers, they flashed through the population; soon most of the people in a community were either dead or immune.

The microbes, having run out of susceptible hosts, died off. Only years, or generations, later could the germ attack successfully again, depending on a new crop of susceptibles and another accident of reintroduction.

Writers often refer to crowd diseases with the metaphor of fire, and of human hosts as the fuel for those fires

If there is too little fuel or if it is too thinly scattered, the blaze sputters out. The image, though simplistic, is basically accurate,

In epidemiologists' terms, a zymotic persists only if the population is dense enough to keep transmitting the germs around and big enough to keep producing new susceptibles.

Herd diseases jump from animals to humans and thrive only if the people form a superherd. Once cities hold several thousand people, they can support most present-day crowd diseases.

We do not know which cities of Mesopotamia were the first to be big enough to sustain the wildfire of various zymotics, each of which had its own population threshold.

Perhaps it was Ur or Nineveh; perhaps it was Babylon, whose name became a symbol of city life's pleasures and perils. Still, the trends of ancient population growth are clear, the major milestones are known.

Modern readers and museumgoers are fascinated by these cities' monuments and art

It is easy to ignore the mundane fact that the gardens of Babylon, the golden mask of King Tutankhamen, and the masonary walls of Machu Picchu all rested on one essential asset: big cities and their culture could not exist without mountains of food; that is, without farmers who produced many times what they needed to sustain themselves.

After people added tin to copper and made tools of bronze, food production could really soar, and urban life could greatly expand.

As happened early in the Agricultural Revolution, health tended to retreat while technology advanced. Another source of illness was the trend toward specialized labor. Many trades and crafts carried distinctive health risks.

In big cities, many workers were exposed to anthrax all day, every day. The anthrax bacillus can survive for years despite heat, cold, or dryness, in infected wool and hides, even in contaminated earth and dust.

Anthrax was only one of many zoonoses to gain a foothold among city workers. For millennia, many people in the Middle East used dog feces in tanning hides; which made tanners and shoemakers prey to echinococcosis, a helminthic (worm) disease often passed from dogs to humans.

Just as virtually every step toward agriculture had invited new diseases, so did each new technology and condition of urban life. Everyone was at risk for various diseases because crowding, filth, and pollution were on the rise.

The gardens of Babylon and temples of Egypt were emblems of urban glory, but the alleys in their shadows were choked with garbage. Homes reeked with fetid air and smoke.

Vast amounts of human and animal wastes accumulated; water was drawn from contaminated wells, food harvested from tainted fields. Dirt and refuse drew every germ-bearing scavener that flew, crept, or crawled.

Puddles, cisterns, and water vessels harbored mosquitoes carrying malaria, yellow fever, dengue, and encephalitis. Granaries and sewers stirred with rats that carried typhus, relapsing fever, hemorrhagic fevers, and perhaps bubonic plague.

Householders caught toxoplasmosis parasites from pet cats. Leptospirosis infection from dogs gave humans a deadly form of meningitis.

The biological price of urbanization, trade, travel, and war increased the saturation of infections across much of the Old World. Technology had allowed diseases to multiply and travel, but it could not yet offer ways to combat them.

Humanity experienced a population crash, of a kind some scientists fear may once again become a part of our future.

Part 3 of 4.