Sadly, however, newborns don’t inherit measles immunity. As a new generation of potential hosts grows, herd immunity fails and measles returns to ravage the population of previously unexposed children. A measles victim is likely to pass on the infection to twelve or more people in an unexposed population, and that means for herd immunity to defeat a measles outbreak, more than 92 percent of the community needs to be individually immune. In turn, that suggests only a few years of new births is required before the proportion of immune people drops below the share necessary to preserve herd immunity and measles can strike again.
The estimate is that to survive and re-infect later generations, measles needs at least five hundred thousand people living in close proximity—otherwise it dies out. And only with an even larger connected population does a disease like measles change from an epidemic (appearing in waves as a new generation of victims grows large enough) to being endemic (always present, infecting the young and previously unexposed). Measles needs large civilizations if it is to survive and thrive.
Three thousand years elapsed between the first evidence of domestic plants and livestock and the beginnings of permanent towns, and about three thousand more counted down before the emergence of the first cities with tributary lands, like Ur and Kish.29 Even these cities began as homes to only ten to twenty thousand people—simply not large enough to sustain a disease like measles.30 But as civilizations grew, measles and smallpox both jumped species to make a permanent home in humans.31
And in larger populations, these new diseases became part of the mundane background of life. The history of smallpox in Japan provides an illustration. By the 1600s, the disease was endemic in Japan’s urban areas. Suffering and recovering from smallpox became a significant part of the ritual celebrating a child’s growth. But in outlying islands with small populations and limited connectivity, the disease remained a periodic, epidemic visitor. And behavior toward smallpox was notably different. Visitors from Japan’s central regions were surprised that islanders would flee from the infection and quarantine victims, often abandoning family members.
According to historian Akihito Suzuki “these behaviors caused emotions ranging from curious bewilderment to moral condemnation of… an unthinkable barbarity” among visitors from the city.32 But the different behavior has an utterly rational component. In endemic environments, smallpox exposure is going to happen—it’s only a matter of when. A level of fatalism is sensible under the circumstances. In communities where a disease only reappears every few years, it might be possible to avoid catching it.
Even if these different reactions may make sense, they probably give us a skewed sense of history. We tend to view irregular, concentrated mortality differently from daily, small-scale mortality (think of deaths from plane crashes versus car crashes—the first garner more attention, the second kill more people). And we’re dulled into inaction by frequent association: how else to explain the fact that most with access to the seasonal flu vaccine don’t get it despite the flu’s killing more than half a million people per year worldwide.33 On the other hand, people notice epidemics and pandemics, calling them plagues. Potential victims run from them. Chroniclers record them and poets bewail them. Sometimes they help end empires. Everyday endemic infection is hardly worth a stanza, let alone flight.
Nonetheless, over the millennia it was the regular, endemic infections (like malaria and—in larger communities—smallpox and measles) that killed greater numbers than dramatic plagues and pestilences. Day-to-day unfriendly local infections felled a third or more people before they reached adulthood. And, in the end, they had the larger role in shaping societies and economies.
We can see the massive health burden that is created when previously unexposed populations are presented with the full diversity of agro-urban disease. An analysis of 238 recently contacted Brazilian indigenous societies from the Amazon basin suggests their population numbers fell an average of 43 percent in the nine years after sustained contact with outsiders from the connected villages and cities of modern Brazil.34
Again, archaeologists have dug up skeletons from peoples who made the transition from hunting to agriculture, and the bones of the agricultural groups are in worse shape than those of their hunter forebears. The bones have lesions in them: signs of infection. Anemia—low oxygen-carrying capacity in the blood, often linked to infection—frequently leaves its mark in spongy bones. And the teeth have low levels of enamel, associated with an unhealthy childhood.
But the liabilities of the agricultural lifestyle don’t stop there. Archaeologists with the stomach to study preserved human feces (coprolites) find more intestinal parasites preserved within them as human groups move through the agricultural revolution.
Alongside more infection risk, people in early civilizations experienced worse nutrition. The field-tending masses became more reliant on a few staple crops and ate less protein. We know broadly what their diet must have looked like because of its likely similarity to the diet of today’s very poorest people. Take the Aboubakar family of Breidjing Refugee Camp in Eastern Chad, interviewed by Peter Menzel and Faith D’Aluisio a few years ago for their book Hungry Planet. The family benefited from the fact that relief agencies were supplying them with staple foods, but their meals were still immensely monotonous. The Aboubakars ate forty-four pounds of grain—mostly sorghum—each week. They ate five pounds of beans, two and a half pounds of vegetables, alongside a couple of quarts of cooking oil and some sugar. But the six family members split just nine ounces of goat and six ounces of fish for the week. The only fruit was five limes (less than one each). They ate no dairy at all.
At least this variety of calories was sufficient to stave off wasting—many poor people outside refugee camps do even worse. Take this description of daily life from a person living in poverty in Vietnam: “In the mornings, eat sweet potatoes, work. At lunch, go without. In the evenings, eat sweet potatoes, sleep.”35
In the earliest civilizations (again, as with the poorest people alive today), the meat that people did get to eat was more likely to be infested with the types of parasite which enjoy living in humans for part of their life cycle. And malnutrition and infection reinforce each other. If you don’t get enough vitamins and minerals, you’re more likely to get sick. For example, vitamin A deficiency increases the risk of diarrhea, malaria, and measles, while zinc deficiency reduces the overall effectiveness of the immune system.36 And parasites, in particular, drain nutrients from human hosts—many worms absorb them directly from inside the intestine.37 Over the millennia, people literally shrank under the combined influence of infection and malnutrition—women by as much as four centimeters on average from their prehistoric height.
The process didn’t reverse until very recently: the depths of human stature probably weren’t reached until the hellhole cities of the Industrial Revolution.38 In 1841, Liverpool saw an average life expectancy of less than twenty-six years—that’s at or below the rate estimated for life expectancy in tropical Africa at the time.39 In 1842, the average age of death in Manchester was just seventeen years old for laborers. That compared to fifty-two for gentry in the countryside area of Rutland and thirty-eight for rural laborers. A mean age of death of seventeen is considerably lower than estimates for hunter-gatherers in prehistory.40
The effects of civilization were particularly grim for women.41 The mélange of infections produced by urbanization and agriculture demanded high birth rates to keep up with the high rates of childhood mortality.42 Accordingly, the average woman for most of recorded history spent a considerable proportion of her life from adolescence to menopause pregnant or breastfeeding.43
In turn, that often reduced women’s autonomy. Many Stone Age groups saw some level of equality between the sexes in terms of roles and decision-making. But the Code of Hammurabi, sixth king of Babylon, suggests how times changed with civilization. Hammurabi, who ruled Mesopotamia around 3,770 years ago, created one of the earliest known sets of laws. These precepts
treated women only a little better than property: “If a man strike a free-born woman so that she lose her unborn child, he shall pay ten shekels for her loss.… If the woman die, his daughter shall be put to death.”44
* * *
The range of diseases that civilization enabled was effective in keeping human population below the carrying capacity of the land.
The Danish economist Ester Boserup challenged Malthus’s ideas about land limits and food supply in her 1965 book The Conditions of Agricultural Growth.45 She pointed out that a population increase may expand the land area used for agriculture, but more significantly, it leads to intensification—producing more food from the same land. One simple change was to redirect land being used for pasture toward growing crops.46 And she notes that techniques to increase the amount of food per acre, like crop rotation, were known long before they were widely introduced.47 Through most of history and in most places, Boserup suggested, neither technological barriers nor lack of land prevented greater output.
Earth scientist Jed Kaplan and colleagues suggest that less than one-half of the land currently used for food production was used in 1600 and less than one-third in 100 CE.48 It is true that making more of the land available takes more work—sometimes brutally hard work, and that risks malnutrition. Again, some land couldn’t be cultivated without innovations, including heavy plows and irrigation. Nonetheless, it seems clear that throughout most of history the number of humans on earth fluctuated far below the maximum possible.
Instead, we should probably thank (or blame) the regulatory mechanism of infection for limiting populations. As the number of people grew, population density drove up disease rates. This thinning mechanism was, in most places, probably the most powerful check on the number of people, particularly during the centuries that humans have been farmers.
Evidence of the relative role of different Malthusian checks is provided by what we know of the history of mass starvation. Cormac Ó Gráda in his Famine: A Short History provides data and descriptions of some of the world’s worst famines. Two points are worth making: first, most are associated with war or a run of bad weather (usually drought, less often rains and floods). It’s not that the usual productive capacity of the land comes close to subsistence, it’s that an extreme shock drives productivity far closer to or even below subsistence. Second, even the most deadly of famines he lists—Ireland in 1740–41 and 1846–52, China in 1877–79 and 1959–61, Cambodia in 1975–79—killed a maximum of “only” 15 to 25 million people, or 13 percent of the population. Pandemics like the Black Death managed far worse over far larger areas, and endemic diseases like smallpox take a far larger regular toll over the long term.49
Again, the degree to which population is driven up or down by infection—as opposed to malnutrition and starvation—is demonstrated by the recorded health history of the rich. Wealthy people can afford better diets. And yet Walter Scheidel’s study of Roman emperors who died natural deaths as well as senators and their families suggests life expectancies at birth for the elite were less than thirty years.50 This was a group famed for its dining habits. The Roman cookbook De Re Coquinaria lists recipes for pheasant, goose, peacock, chicken, flamingo, parrot, crane, duck, wood pigeon, squab, figpecker, partridge, turtle dove, woodcock, and ostrich, to mention some of the birds alone. Full stomachs didn’t seem to confer much longevity.
Throughout most of history, to keep population-dense cities like Rome occupied took a constant stream of rural migrants—because urban populations couldn’t produce enough children to keep up with death rates. Especially in early civilizations, that migration was rarely voluntary. James Scott argues in his Against the Grain that ancient states “replenished their population by wars of capture and by buying slaves on a large scale from barbarians who specialized in the trade.”51
The largest cities drew in new victims from hundreds of miles of hinterlands and boasted comparatively advanced sanitation, but still they couldn’t be sustained over the long term. If war or imperial collapse removed the power and ability to attract migrants, urban populations could rapidly collapse. According to Tertius Chandler, the population of Athens halved between 430 and 100 BCE as the city was eclipsed by Rome, and in turn the population of Rome fell by nearly 90 percent between 100 CE and 600 CE as the empire began to fall apart.52
To be absolutely clear, most of history has still seen Malthusian outcomes at a local level: as populations rise, average incomes and consumption fall. But it appears that only at times of crisis, sometimes linked to changing climate, has lack of food become the binding constraint to population numbers. Ester Boserup was right: it wasn’t shortage of land. What kept populations low and dispersed was a high death rate driven by infection. And when infectious death declined in the nineteenth and twentieth centuries, population, urbanization, intensification, land use, and prosperity all climbed to historically unprecedented levels worldwide.
CHAPTER THREE Trade Merges Disease Pools
… a pestilence, by which the whole human race came near to being annihilated.
—Procopius
The plague of the Israelites—supposedly a heavenly punishment for running an improper census. (Credit: The Plague of the Israelites. Engraving by E. G. Petit, 172–, after P. Mignard. Wellcome Library, no. 6346i)
Agriculture and cities are sedentary—people largely stay where they are to manage fields, markets, and temples. And, historically, that created the need for trade: if you couldn’t move to where the goods were, the goods would have to move instead. James Scott has suggested that the early state centers in Mesopotamia traded for wood, leather, obsidian, copper, tin, gold, silver, and honey.1 But with the goods (and even more so, the slaves) that were transported by boat, horse, and porter came the diseases of foreign lands. Add the migration of troops in warfare and the first steps on the pathway to a global disease pool had begun.2
Ancient Greek city states paid a high price for this growing connectedness. They were particularly susceptible to foreign diseases because they were so reliant on trade. The country’s thin limestone soil received little rain, so its population clustered on the coasts, where the sea offered sustenance. To supplement their diet of fish, Athenians imported grain from the Crimea, Sicily, and Alexandria. In some years, as much as 3 million bushels—more than forty Olympic swimming pools’ worth—were imported through the Black Sea alone.3
The plague of Athens was one of the earliest reliably recorded pandemics. According to the chronicler Thucydides, who suffered from the plague himself, it spread through North Africa and then invaded Athens in 430 BCE. Despite Thucydides’s detailed description, argument rages over what the disease was—one strong candidate is typhoid; Ebola is also a contender.4 But we don’t recognize the collection of symptoms described by Thucydides as a single modern infection, perhaps because it has evolved since then.
People in good health were all of a sudden attacked by violent heats in the head, and redness and inflammation in the eyes, the inward parts, such as the throat or tongue, becoming bloody and emitting an unnatural and fetid breath. These symptoms were followed by sneezing and hoarseness, after which the pain soon reached the chest, and produced a hard cough. When it fixed in the stomach, it upset it; and discharges of bile of every kind named by physicians ensued.… The patient could not bear to have on him clothing or linen even of the very lightest description; or indeed to be otherwise than stark naked.… They succumbed… in most cases, on the seventh or eighth day to the internal inflammation.… But if they passed this stage, and the disease descended further into the bowels, inducing a violent ulceration there accompanied by severe diarrhoea, this brought on a weakness which was generally fatal.5
In part thanks to the plague, Athens lost its war against neighboring Sparta. Not for the last time, disease was a force for considerable social and economic upheaval. The great academic Arnold Toynbee, author of the twelve-volume Study of History, was overwrought on the subject, but he saw the glories of democratic Athens gi
ving way to an authoritarian utopianism bolstered by the intellectual backing of the philosophers Plato and Aristotle:
In Plato’s Utopias and Aristotle’s alike… the aim is not the happiness of the individual but the stability of the community. Plato… advocates a general censorship over “dangerous thought” that has its latter-day parallels in the regulations of Communist Russia, National-Socialist Germany, Fascist Italy and Shintoist Japan.6
We will see the type of authoritarian control and xenophobia of which Toynbee accused the Greek philosophers frequently accompany disease outbreaks up to and including border closing responses to the Ebola outbreaks of 2014 and the fast-moving Covid pandemic in 2020.
In the same century as Athens was weakened by plague, the Roman historian Livy reports his city was fighting the Aequi and Volscians when an outbreak struck:
The violence of the epidemic was aggravated by the crowding into the city of the country people and their cattle through fear of raids.… Their being brought into contact with each other in ordinary intercourse helped to spread the disease. The mortality in Rome through the epidemic was not less than that of the allies through the sword.… The Senate, deprived of all human aid, bade the people betake themselves to prayers.7
The Plague Cycle Page 3