Nosology and why diseases expanded around the world

Cities have inspired a dismal narration of decay and misfortune

From Petronius to Dickens and Zola, writers have seen the metropolis as a warren of corruption, poverty, violence, and disease. The very existence of cities seems to bring fear that the hubris of creating them will be punished by disaster and collapse.

• They evoke images of the swamp and the jungle.
• 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 to crowd 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.

Farms and villages started to increase the incidents of death after the nomadic movements of humans

• For several million years, the main causes of nomadic human deaths were accidents and wounds.
• Permanent farms and villages made death by disease far more frequent.
• Then the population explosion of the Bronze age, 6,000 years ago, took city dwellers beyond a crucial threshold.
• Urban masses became sufficiently large and dense to support zymotics, or crowd diseases, what in other species are called herd diseases.
• For the first time, infection became humanity's chief cause of death.
• Despite a few respites, this would remain true in the West until this century.
• Infections are still the main killers in many poor nations, and they recurrently threaten the rich ones.
• The reason epedemics did not take hold until urban times is simply the conditions imposed by the grouping of numbers of people.
• While nomads were not free of infection, their most common diseases were chronic, not acute.
• Deadly epidemics remained relatively limited and infrequent for the same reasons they had been so among hunger-gatherers.
• People did not live densely packed together which facilitates the transmission of germs from one person to another.
• Their settlements were sufficiently far apart, and travel was sufficiently limited, to keep outbreaks of diseases localized.

Writers often speak of crowd diseases with the metaphor of fire and of human hosts as fuel

• 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 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.

Unit about diseases and sickness at noso-, -nosis.