I The Pitiless War Infectious disease is one of the great tragedies of living things--the struggle for existence between different forms of life.Incessantly the pitiless war goes on, without quarter or armistice. --Hans Zinsser, Rats, Lice and History, 1935 Visitors from the Deep Past For untold generations, before the invention of written history, people lived in small bands of relatives numbering, at most, a few dozen members. Our distant ancestors were merely creatures among other creatures, struggling to survive in an untamed wilderness. Called "hunter-gatherers" by modern social scientists, they were nomads, wanderers, people without a fixed place to live or call home. Each band had little contact with other bands, going from place to place, hunting animals and gathering roots, nuts, berries, and fruits to eat. Unable to preserve or store food, nomads had to move continually, and on foot, to find their next meal. Without the wheel, a later invention, they also lacked draft animals; they kept no animals, except dogs, used for hunting and, in a pinch, for a meal.
They carried their few possessions strapped to their backs or lashed between two wooden poles, which the women dragged along the ground. The able-bodied men walked ahead, armed with clubs, stone-tipped spears, and bows and arrows, eyes peeled for danger or for game to pursue. Camps usually were just overnight stops to eat and sleep. But if the hunting in an area was good, the band might stay for a few days longer to butcher a kill, fill their bellies, and rest up for the trek ahead. Hunting accidents, wounds, falls from trees, feuds within a band, and clashes with other bands took a steady toll. Still, the nomadic lifestyle had one advantage: it limited the impact of infectious diseases carried by animals. All types of living beings have diseases that afflict them alone. Sometimes, however, a disease attacking one life-form "crosses over" and infects another life-form.
Ancient nomads did not live amid heaps of rubbish, their own waste, and polluted water. After a few days at a campsite, they moved on, leaving behind any disease-causing microbes that might be around. If a disease crossed over to, say, a hunter, he might die. The disease might even infect the entire band, killing everyone. But that would be the end of the disease; it stopped when there was no one left to infect. It could flourish only by becoming a "crowd disease," infecting a population large enough to allow victims to pass it to the healthy.1 About 11,000 years ago, humankind reached a critical turning point. Across the world, big game animals--mastodons, giant sloths, and saber-toothed tigers--became extinct, probably due to over-killing by the hunters themselves.
Naturally, as food became scarcer, nomads sought other ways of feeding themselves. Many began to experiment, growing wild plants like wheat, barley, and rice for food. They also learned to domesticate wild animals; that is, to tame and raise them. Cattle, horses, oxen, sheep, goats, pigs, ducks, geese, and chickens: all became important food sources, and some, like horses and oxen, became work animals. This turning point, called the Agricultural Revolution, placed new demands on people. Above all, it required the cooperation of several groups living close together. Of necessity, hunter-gatherer bands settled into permanent communities when they took up farming. With a larger, more reliable food supply, their numbers grew.
Over the centuries, individual farms linked up to form villages, villages grew into towns, and towns into cities. The first cities arose in the fertile valleys of the Tigris and Euphrates rivers in what is today Iraq, and along the Nile River in Egypt, the Indus River in India, and the Yellow River in China. Farming and cities later emerged in the New World, chiefly in Peru and Mexico, based on crops like maize and the potato. Civilization is the product of cities. Farmers usually grew more food than they needed, and the surplus gave others the leisure to do other things. Craftspeople, artists, priests, architects, engineers, and astronomers thrived. Over the centuries, they invented writing and mathematics, studied the sky, and created the calendar, which enabled farmers to plant and harvest at just the right times. Eventually, rulers raised armies and created empires to expand their domains.
The Agricultural Revolution, however, was a mixed blessing. Author Jared Diamond has even called it "the worst mistake in the history of the human race." Agriculture, Diamond argues, was harmful to health in several ways. Archaeologists, scientists who study early peoples through their physical remains and the things they built, have found that more food did not always mean better nutrition. Preserved teeth and bones show that the common people, the vast majority, were less healthy than their hunter-gatherer ancestors. A largely plant-food diet is high in sugars and starches but low in proteins, the chemical building blocks of life. This meant that the masses of farm-folk were shorter than their ancestors--down from an average of 5''9" to 5''3" for men, and from 5''5" to 5''1" for women. Hunter-gatherers also lived longer, up to about forty years.
Because of their diet and longer hours of strenuous work, farmers were usually old at twenty-five and dead by thirty.2 Yet there were other culprits. To clear the land for planting, farmers cut down forests, plowed the soil, and dug irrigation canals. These activities displaced native bacteria from their environments, where they were harmless to humans, and created pools of stagnant water, breeding places for disease-carrying mosquitoes. Irrigation canals also allowed microscopic worms such as blood flukes to enter the bodies of anyone who walked in them barefoot. Dried worm eggs have been found in Egyptian mummies 3,000 years old.3 Farm settlements became magnets for infectious diseases in other ways. Grain mills and storehouses attracted hordes of rats, mice, and insects.
To make matters worse, farmers lived close to their animals--close to their feces, urine, blood, breath, blisters, vomit, sweat, sores, spittle, and snot. To discourage thieves, they might take prized animals into their homes. Farmers also collected human and animal waste to spread on their fields as fertilizer, or to tan hides into leather. Thus, by forcing people and animals to live close together, the Agricultural Revolution created ideal conditions for crowd diseases to take hold. Close contact enabled the microbes that cause certain animal diseases to cross over to human hosts; hosts are living beings, animal or plant, on which or in which another organism lives. Today we share no fewer than 300 diseases with domesticated animals. For example, humans get 45 diseases from cattle, including tuberculosis; 46 from sheep and goats; 42 from pigs; 35 from horses, including the common cold; and 26 from poultry. Rats and mice carry 33 diseases to humans, including bubonic plague.
Sixty-five diseases, including measles, originated in man''s best friend, the dog. We can still get parasitic worms from pet dogs and cats. That is why it is not a good idea to kiss a pet on the mouth or sleep with it in bed.4 Enclosed by high stone walls, with houses jammed close together along narrow winding streets, ancient cities were even more prone to crowd diseases than farms. Sanitation services did not exist, and drains flowed into the cobblestone gutters. City dwellers threw human waste into the streets, where it rotted, stank, and attracted vermin. Gutters ran with urine and liquefied manure. Stockyards and animal holding pens, usually located in crowded neighborhoods, swarmed with flies, fleas, and lice.
Butchers slaughtered animals outdoors, in front of their shops, leaving puddles of blood; waste like brains and guts wound up in streams used for washing and in drinking water. So it is no accident that, for thousands of years, animal-borne diseases killed more city dwellers than were born each year. For that reason, cities needed a steady inflow of immigrants from the countryside to bolster their populations. Country-folk came seeking adventure and opportunity. Civilization''s Plagues All ancient civilizations suffered from infectious diseases that crossed over from animals. The Old Testament tells how the Lord threatened to send plagues to the Egyptians unless Pharaoh, the ruler of Egypt, released the Hebrews from bondage. When Pharaoh refused to change his mind, the Almighty caused "sores that break into pustules on man and beast."5 Pus is a yellow-white liquid that the body produces during infection; it consists of dead white blood cells and bacteria.
The first detailed description of an urban plague comes from ancient Greece. In 430 B.C., the city-state of Athens went to war with the city-state of Sparta. When the Spartan army invaded Athenian territory, thousands of farmers from outlying villages fled to the fortified city with their livestock. Already overcrowded and filthy, Athens became even more so. Within days of the refugees'' arrival, a disease more terrible than anything ever experienced broke out. The historian Thucydides, an eyewitness, described how Athenians died horribly, the sickness beginning with "violent heats in the head," followed by furious coughing, vomiting blood, and finally severe diarrhea.
The Plague of Athens killed young.