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But it wasn’t a one-way relationship from globalization to infection. Just as disease set the limit to urban growth, it also limited imperial growth. That’s because, from the earliest empires, armies campaigning far from home were particularly susceptible to disease—both local microbes as well as the infections that always haunted large masses of people congregated together without adequate sanitation.
For the vast majority of history it was this infection, far more than wounds suffered in battle, that killed off the most troops. To quote the Harvard biologist Hans Zinsser from his entertaining and path-breaking study of the impact of infectious disease on history written in 1935:
In point of fact, the tricks of marching and of shooting and the game called strategy constitute only a part—the minor, although picturesquely appealing part—of the tragedy of war. They are only the terminal operations engaged in by those remnants of the armies which have survived the camp epidemics.44
The wise men of the ancient world were well aware of the risk of unsanitary camp conditions. Deuteronomy (23:12–14) reports that Moses was fastidious when it came to keeping camps clean, for example: “Thou shalt have a place also without the camp, whither thou shalt go forth abroad: And thou shalt have a paddle upon thy weapon; and it shall be, when thou wilt ease thyself abroad, thou shalt dig therewith, and shalt turn back and cover that which cometh from thee.”
But concern with camp cleanliness wasn’t enough to stop huge numbers of troops throughout history from dying of infection. For example, three different crusades were stymied by three different illnesses: plague, dysentery, and typhoid. Between 1098 and 1099, a Christian army that took Antioch and Jerusalem was reduced from three hundred thousand to sixty thousand men, largely by infection. And nearly all of the half-million-strong army of the Second Crusade perished in a similar manner.45
It wasn’t just—or even mostly—the troops: one example of the carnage wrought by the microscopic camp followers of far less deadly armies is Europe’s Thirty Years’ War in the seventeenth century, sparked by religious conflict in Germany. In France, mortality rates may have as much as doubled, and the Holy Roman Empire itself lost 5 to 6 million people—more than a third of its population.
Along with plague, typhus is one of history’s biggest killers. It emerged in epidemic form at the end of the fifteenth century, and rapidly became a particularly effective (if indiscriminate) weapon of military destruction in conflicts including the Thirty Years’ War. Typhus is spread by lice, including Pediculus humanus corporis—also known as the cootie or clothing louse. As an infected louse feeds on its host, it defecates bacteria-laden feces, and when the victim scratches their itching bite, that action rubs the feces into the wound.
As bad as this is for the host, it’s no picnic for the louse either, as the sympathetic Hans Zinsser relates:
If lice can dread, the nightmare of their lives is the fear of some day inhabiting an infected rat or human being. For the host may survive; but the ill-starred louse that sticks his haustellum through an infected skin, and imbibes the loathsome virus with his nourishment, is doomed beyond succor. In eight days he sickens. In ten days he is in extremis, on the eleventh or twelfth his tiny body turns red with blood extravasated from his bowel, and he gives up his little ghost.46
Typhus bacteria breed in the lining of small blood vessels. The infected cells slough off, block the vessels, and stop the flow of blood—starving surrounding tissues of nutrition and oxygen and breaking the vessels open. The condition begins with chills, fever, headache, and a rash, followed by back pain, coughing, insomnia, and—in terminal cases—delirium, coma, and death. In the Thirty Years’ War, typhus was spread by desperate, dirty, and ravaging armies short of food—ones even more than usually despising of local populations on the wrong side of the religious divide.47
The threat of disease was also one reason why European colonization in the tropics, in particular, was so limited until the nineteenth century. In 1805, Scottish explorer Mungo Park led an expedition to Timbuktu (in modern Mali), and all but two of his forty-strong team of European expeditionaries died of infectious diseases. That was a far from unusual outcome. Death rates for British troops stationed at home in the 1820s were around fifteen for every thousand troops. In India, they were twice to five times as high. In the West Indies, they were six to nine times as high and in West Africa, thirty-two to forty-five times as high. Around half of those stationed in West Africa died each year—nearly all from disease.48 The risk of death was only worth it if there was potential for massive returns (in the case of the sugar industry), or limited contact (in the case of the slave trade from Africa). It simply wasn’t possible to set up large, centrally managed colonies across most of the tropics because soldiers would die too quickly to retain control.
Emperor Napoleon was perhaps the last great imperialist to see world-spanning ambition dashed by the microbe. He successfully imposed his rule (or satellite status) on Spain, Switzerland, the Italian states, Prussia, Sweden, and Austria, and extended the borders of France as far as Denmark in the north and through the Italian Piedmont in the south. But his ambitions spread even farther—Napoleon had dreamed of an empire stretching from the bayous of Louisiana to the steppes of the Russian plain and the upper reaches of the Nile. At all three geographic extremities, disease destroyed his armies. In 1798, General Napoleon invaded Egypt and Syria. The adventure was not his finest military hour. Plague killed thousands of his troops and helped end a campaign already marred by poor planning and lack of equipment. Egypt was returned to the Ottomans with the support of the British army in 1801.
In the same year, Napoleon sent his brother-in-law, Victor-Emmanuel, along with twenty thousand troops to Haiti to depose Toussaint Louverture, a former slave who’d risen up against imperial rule a few years before. The emperor wanted to use Haiti as a base for the creation of a Mississippi Valley colony. General Victor-Emmanuel Leclerc won some resounding early victories, but between January and April of 1802 he lost his own life, the lives of the considerable majority of his original troops, and more than thirty thousand reinforcements to yellow fever.49 By 1803, the battle for Haiti had been lost. Napoleon abandoned his American colonies and sold the territory of Louisiana to the United States for the bargain price of $15 million.
For another decade, Napoleon had considerably more luck in Europe. By 1812, the empire and its dependencies stretched from Spanish Algeciras to Warsaw. But his campaign against Russia in that year was thwarted in large part by typhus. Of the half million or so troops that had crossed the Nieman River into Polish Russia in June, perhaps twenty thousand re-crossed it in December.
By the time of the Napoleonic Wars, typhus was a well-known threat. As the emperor marched through Poland, his army surgeons warned him that the local population was rife with the illness. Napoleon ordered no contact between his soldiers and Polish people. But the army’s supply train was foundering behind, along roads ill-designed for heavy wagons. The orders were ignored as troops went in search of something to eat.50
Jakob Walter, a stonemason, had been drafted into Napoleon’s army. He later wrote an autobiography in which he reported his experiences in the Russian campaign. He suggests that troops were already hungry enough to be slicing meat off live pigs and eating it raw as they reached the Russian border. Three days into Russia, marching through a swamp that provided nothing in the way of forage or fuel, Walter “lay in the tent shelter cold, hungry and wet.” He was thankful soon enough that comrades “who came in and lay down upon me served as warm cover.”51 So, no doubt, were the lice that accompanied them.
The emperor’s surgeon general, Dr. Dominique-Jean Larrey, reported that sixty thousand men were judged sick by their commanding officers, and the true figure was probably twice as high.52 Dysentery, hepatitis, and a host of other conditions played a role, but it was typhus that was expanding to epidemic proportions. By mid-August—only two months into the campaign—the French army’s effective
fighting force was only a little more than a third its size in June, and that attrition had occurred before there’d been a single major military engagement.53
On September 7, at Borodino, the French and Russian armies finally met. By the end of that day, writes Stephan Talty in his history of the campaign The Illustrious Dead:
The French had lost 28,000 men… the Russians, about 45,000, roughly half of their entire frontline troops.… It was the deadliest engagement in the annals of warfare to that date. It would take a hundred years, until the battle of the Somme, for the totals to be exceeded.
But while the Russians retreated in the end, Napoleon failed to destroy their army, or dent the Russians’ commitment to the war. Typhus had taken from Napoleon the troops he’d needed for decisive advantage, argues Talty “and with them the battle, the war, and the future of the empire.”54
Imagine the scenes in the army hospitals of the French after Borodino: Surgeon General Larrey himself performed two hundred amputations in the day after the battle, rarely pausing to wipe off instruments between each cut, or even each patient. Wounds to the chest or stomach were usually considered simply too dangerous to operate on. Patients were left to recover—or more likely die—where they were. For those disemboweled by cannon shot, the mess of intestines was cleaned off as best as possible, stuffed back into the body, and covered with a linen bandage. The room would have stunk with the fecal matter of those who’d lost their digestive tracts and those who’d merely lost control of them—that along with the fetid smell of gangrene. The air would have been filled with screams of un-anaesthetized victims losing a limb to a surgeon’s blunt saw, its teeth likely still clogged with the cartilage and bone of the last patient. And around the walls, delirious typhus victims in the last stages of the disease would have added to the din. As those sufferers died, an army of lice, carrying the disease with them, would have found new hosts—dooming many of those lucky enough to avoid serious infection from wounds or the surgeon’s saw.
Napoleon marched onward to Moscow. Foot soldier Jakob Walter reports that there were beets and cabbage aplenty to be foraged around the city, as well as a break in the cold weather, but neither lasted long.55 The army camped with no sign of Russian surrender. After weeks more of losses to disease, malnutrition, and encroaching starvation, the emperor ordered retreat, abandoning thousands of the sick to the mercies of the returning Russian government. At the start of the long march back to Germany, he was reduced to seventy-five thousand troops, less than one-sixth the force he’d had at the start of the campaign.
As the army made its way through poor agricultural lands already stripped of what had been available on the march in, as the temperatures dropped, and as Russian troops harried the retreating columns day after day, the situation became increasingly desperate for all but the parasites. Walter writes that “the fighting, the shrieking, the firing of large and small guns, hunger and thirst, and all conceivable torments increased the never-ending confusion. Indeed even the lice seemed to seek supremacy, for their number on both officers and privates was in the thousands.”56 At the request of his major, Walter tried to kill the lice in the officer’s shirt collar, “but when I had his collar open, his raw flesh showed forth where the greedy beasts had gnawed in. I had to turn my eyes away with abhorrence.” Recognizing that his own body was similarly infested, he tried to comfort himself with the aphorism that “lice stay on healthy people only.”57
Vilnius, the capital of Lithuania, was toward the end of the retreat. Only twenty-five thousand troops reached the town. And only three thousand were to leave it. In August 2001, construction workers in Vilnius were tearing down an old Soviet army barracks. As they cleared the foundations, the workers came across a mass grave—seven corpses in each square meter of ground over an area almost twenty meters a side—two or three thousand bodies in all. The corpses were still dressed in fragments of uniform from the French Imperial Army. Examining the bodies, researchers from the University of the Mediterranean in Marseilles looked for evidence of what had killed them. The scientists found that fully one-third had been infected by louse-borne diseases.58
Walter reports that as he traveled back home through Wurttemberg as one of the few survivors “we were shunned like lepers” and locked in a house together. It did not matter—as many as 250,000 died in Germany from the typhus brought back from Poland and the Pripet Marshes.59 Walter himself recovered from a fevered illness thanks, he suggests, to a course of vinegar and bleeding. Invalided out of the army, he lived at least until 1856—a very lucky exception to the vast majority of Napoleon’s invading force. Napoleon himself fought on for three more desperate years until his final defeat at the battle of Waterloo, his dreams of global empire ultimately reduced to Longwood House on the prison island of St. Helena in the South Atlantic.
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If civilization created a firestorm of disease, disease also set the limits to the scale of urbanization. And if globalization merged disease pools, the pools crucially shaped the nature and extent of colonization and commerce. Throughout most of history, humanity’s only major effective response to infection—exclusion—helped ensure those limits remained. Only with the sanitation revolution was urbanization and integration freed from the constraints of infection, and only with vaccination and antibiotics did that process go global.
CHAPTER FIVE The Exclusion Instinct
Reports of illegal immigrants carrying deadly diseases such as swine flu, dengue fever, Ebola virus, and tuberculosis are particularly concerning.
—US Representative Phil Gingrey, 2014
Plague victims fleeing London are told to “Keepe out” by locals. (Credit: H. W. Haggard, Devils, Drugs and Doctors. Wellcome Collection. Attribution 4.0 International [CC BY 4.0])
When it comes to evolution, the survival of the fittest applies as much to the risk of being killed from within as from without. And this has had an impact on both biology and behavior. Humans have inherited and evolved biological defense mechanisms to counter microbes, but they’ve also evolved behavioral responses, including a preference for cleanliness and a distaste for the diseased. Exclusion and sanitation both build on those behavioral traits.
To begin with the biological defenses, sex may be one early response to infection. If you budded from your mother, you’d be her identical twin and susceptible in exactly the same way to exactly the same infections as she was. Sex mixes up genes—some of which might help defend against particular diseases. That makes it less likely one parasite can wipe out a whole population.1 (And in the case of Covid-19, the sex of the infected person may make a difference to mortality rates, with men dying more frequently than women among early cases.)
Another evolutionary response to disease is the white blood cell. AIDS—acquired immunodeficiency syndrome—gives us an idea about what life would be like absent those infection-fighting cells, because the disease destroys them. AIDS sufferers experience very low white-blood-cell counts, and rapidly acquire a range of infections as a result. Pneumonia and tuberculosis (the blood-coughing lung disease) are frequent AIDS fellow travelers. The HIV virus that causes AIDS uses our defense mechanisms against us: one evolved response against infection (sex) to attack another (the white blood cell).
As a cause of most human deaths over thousands of years, infection has also fostered genetic diversity among the world’s human populations. This diversity isn’t by any means the primary factor in whether a person dies from an infectious disease or survives it. The rate of evolution of rapidly reproducing viruses and bacteria is distinctly quicker than the rate of human evolution, suggesting the lethality of various infectious diseases probably has a lot more to do with their evolution than with ours. That’s one reason it’s so difficult to know what diseases people are describing in the distant past: they’ve often evolved since then. But people whose ancestors lived many centuries in areas that were hospitable to particular diseases have sometimes inherited somewhat more effective biological responses to those di
seases.2
Take malaria: it’s caused by a tiny protozoa, a single-celled organism. Carried by the mosquito and injected with the saliva that the insect uses to prevent clotting as it sucks blood, the parasite travels through the bloodstream to the liver, where it grows and multiplies. Eventually the malaria organisms burst out of the liver cells that have nourished them and re-enter the bloodstream, invading red blood cells, feeding and multiplying again, before destroying cells in a coordinated attack and floating onward to infect another—or to be sucked up into the gut of a mosquito feeding on the prostrate body of the malaria victim.
As the millions of red blood cells collapse, the human host shivers even as their temperature shoots up. The host suffers blinding headaches. If they’re lucky, the fever breaks in a few hours—at least until the next round of blood cells collapses under the onslaught. If the host is unlucky, enough infected blood cells crowd the brain to block the transfer of oxygen. The person becomes unconscious, enduring muscle spasms that weaken as he or she slips toward death.
Malaria used to be present in countries as far north as the UK.3 But there are different variants of the protozoa, some more lethal than others. And falciparum malaria, the most deadly variant, is concentrated in Africa between the Sahara and South Africa. Some people in that region developed a costly genetic defense against the disease—the sickle cell trait. Red blood cells in people who inherit the trait from one parent form a distinctive sickle shape when oxygen is limited, and that apparently helps them resist the malaria parasite. But in people who inherit the genetic trait from both parents, blood cells constantly form in the sickle shape, which can lead to anemia, and increases the risk of stroke, organ damage, infection, and heart failure.
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