Germs, Genes, & Civilization: How Epidemics Shaped Who We Are Today

by David P. Clark

  Little information seems to be available on the precontact rate of infant mortality or its causes. Several respiratory and gastrointestinal infections that can kill infants but have only mild effects on adults probably were in circulation in America and were carried back to join the large number of similar infections already circulating in Europe without attracting much attention.

  Indigenous American infections

  Examination of pre-Columbian corpses from tropical areas of the Americas has shown moderate levels of infestation with parasitic worms and protozoa. However, inhabitants of tropical regions of Africa or Asia typically have much heavier parasite burdens than precontact Americans. In particular, the three most fearsome tropical diseases, malaria, sleeping sickness, and yellow fever, were absent from the Americas.

  Chagas disease is found only in South America and is caused by the protozoan Trypanosoma cruzi, a relative of the trypanosome that causes African sleeping sickness. Chagas disease causes fever, chills, muscle pain, and nosebleeds. Many recover after this initial phase, but other victims develop a chronic disease of the internal organs, especially the digestive tract and heart. Today about 17 million Latin Americans are infected, and approximately 50,000 die each year. DNA sequences specific for Trypanosoma cruzi have been identified in mummified corpses that are several thousand years old. The mummies are from the Atacama Desert, found where Peru and Chile meet. This area has served as a burial ground for about 9,000 years for Chilean and Peruvian Indians, who buried their dead in shallow graves. The Atacama Desert is so dry that space scientists use it to simulate Mars. Corpses buried there have been desiccated and preserved, although they were not deliberately mummified. About 25% of the mummies tested positive for Chagas disease, compared to 10% to 15% of the modern-day population of Chile.

  Chagas disease is spread by the kissing bug, which lives in cracks in house walls. This inch-long vampire emerges to suck blood from its human victims at night. The trypanosome is spread when the bug’s droppings infect the wounds it made to suck blood. Charles Darwin probably contracted Chagas disease during his famous voyage around South America on the Beagle. Consequently, he was a semi-invalid later in life. This could be the major reason he settled down to write The Origin of Species instead of gallivanting off around the world on more naturalistic expeditions.

  Another insect-borne South American infection is Carrion’s disease, also known as Oroya fever. Carried by sandflies, it is caused by Bartonella, a member of the degenerate rickettsia group of bacteria (other species of Bartonella cause “trench fever” and “cat scratch disease”). The death rate is about 40% nowadays. Some of the survivors suffer continuing skin lesions, referred to as verruga Peruana. Carrion’s disease is solely a human disease. This contrasts with another indigenous American infection, Lyme disease. This is spread by ticks that normally live by sucking the blood of wild deer. Lyme disease is primarily an animal disease and infects humans only by accident. When humans penetrate woodland areas inhabited by deer, ticks carrying Lyme disease sometimes bite them. Although Lyme disease is debilitating, it is rarely fatal. Currently, Lyme disease is largely located in the northeastern United States, but it is spreading slowly. In pre-Columbian times, sporadic infections with Lyme disease presumably occurred but were of little overall importance.

  Lack of domesticated animals in America

  Pre-Columbian American populations were substantial. Perhaps as many as 20 million to 30 million inhabitants were present in what is now Mexico, 10 million to 20 million in the Inca Empire, and another 10 million to 20 million in North America. Archeological evidence and skeletal samples suggest that these high populations were relatively recent and were severely straining the natural resources available to societies with stone-age technology. As large animals grew scarcer, the people became more dependent on maize. As in the Old World, sedentary agriculture supported a denser population at the cost of a lower-quality diet, with less meat and more cereals.

  When Old World populations crowded together in this manner some thousands of years earlier, a series of epidemic diseases emerged. From an Old World perspective, the New World was overdue for such epidemics. Some evidence suggests that epidemic diseases were beginning to appear. The Aztecs recorded several outbreaks. In particular, a drought followed by famine and disease occurred in the early 1450s, fewer than 50 years before the Europeans arrived. The descriptions are too vague to identify any disease with certainty; however, typhus acting on a weakened population is a plausible suggestion.

  Virulent epidemic diseases cannot emerge from a vacuum. Indeed, most Old World epidemic diseases originated from infections of domestic animals. The immigrants who populated America split off from the rest of mankind before most animals were domesticated and had only the domestic dog to take with them. Very few animals were domesticated after entering the American continent. These were the turkey, Muscovy duck, guinea pig, and llama and alpaca (both from the same original wild progenitor). These animals did not exist in huge herds before domestication, and their numbers remained relatively small afterward. Llamas, for example, never spread beyond the Andes. Large herds of horses and cattle did roam the New World until around 11,000 years ago, when the last Ice Age ended. Whether these herds were exterminated by early Indian hunters or by the changing climate is debatable. In either case, their demise meant a lack of animals suitable for domestication and, hence, the absence from the New World of mankind’s major source of novel infectious diseases.

  The first epidemic in the Caribbean

  Although smallpox normally takes pride of place in the litany of pestilence that struck down the indigenous peoples of the Americas, it was not actually first ashore. In 1494, an epidemic spread from Columbus’s ships and ravaged the island of Hispaniola (today split between Haiti and the Dominican Republic). From there, it was spread to Cuba, Jamaica, and other Caribbean Islands partly by the Spanish and partly by islanders fleeing from Hispaniola. About a third of the Spaniards fell ill, though few fatally. Vast numbers of the islanders died. The identity of this disease has remained puzzling to this day. Malaria, smallpox, yellow fever, and bacterial dysentery have all been blamed, yet none fits the bill. These diseases did assault the Americas in due course and were recognized when they arrived. Several investigators have suggested an intestinal infection of some sort, based on the symptoms observed.

  A recent theory blames influenza carried by pigs taken aboard Columbus’s fleet in the Canary Islands. The extremely rapid spread and the high proportion of people infected are typical of flu. In addition, most of the symptoms would fit with a virulent outbreak of swine flu. However, there are also arguments against this. An influenza epidemic did invade the New World in the 1550s and was unambiguously identified. Furthermore, influenza in the 1500s was still new enough among Europeans that the death rate was around 20%, far higher than seen in 1494 among the Spanish. It is hard to imagine a virus as incredibly transmissible as flu failing to spread to the mainland. Yet the 1494 epidemic does not seem to have moved beyond the Caribbean. It is also hard to imagine that a full third of the Spaniards remained uninfected by the flu until they made landfall on Hispaniola. We would expect most of the Spanish to have been infected and developed immunity during the early part of the voyage, with one or two stragglers barely keeping the virus in circulation until they reached America. Although an aberrant strain of influenza remains a reasonable contender, the identity of the 1494 outbreak is still uncertain.

  Epidemics sweep the American mainland

  The first major epidemic to harry the mainland started when smallpox reached Hispaniola in 1518. From there it was a short step to Mexico, where smallpox arrived in 1520, just in time to save Cortez from an Aztec counterattack. The Aztecs, with overwhelming numbers on their side, had driven Cortez out of their capital city, Tenochtitlan, and things looked bleak for the conquistadors. Smallpox arrived with the Spanish relief expedition, and disease fought alongside the Spaniards. The Aztecs were devastated. Those whom smallpox
did not kill were immobilized by shock. From then on, the legions of Old World viruses raced ahead of the conquistadors. By the time Pizarro reached the Inca Empire in present-day Peru, smallpox had already done its work, arriving in 1525–1526. The ruling emperor and his immediate heir had both succumbed to smallpox, and civil strife over the succession to the Inca throne had ensued. Unlike Cortez, Pizarro met no significant military resistance.

  Rough estimates suggest that around a third of the total population died of smallpox. The death rate was doubtless higher among those crowded in large cities, whereas many smaller, isolated communities escaped the worst effects. This is remarkably similar to the first arrival of smallpox in Japan in the 700s. In the cities, two-thirds or more died, and the overall death toll was about one-third. Thus, there was nothing magical about the effects of smallpox on the Amerindian population. Exposure of Old World populations to new and virulent infections has had much the same effect.

  Measles followed smallpox, and the contrast between European and American susceptibility was even greater. Smallpox killed Europeans, albeit less often than Amerindians, but measles was rarely lethal to those of European descent. The measles epidemic of 1530–1531 raged through the Aztec domains and then followed smallpox into the Inca territory in South America. In 1546, a third epidemic followed whose identity is still uncertain. In any case, eventually 95% or more of the indigenous population of the Americas was exterminated by these successive epidemics.

  Interestingly, the next major epidemic was rather different. Influenza ravaged Europe in 1556–1560. An estimated 20% of the population died in England, and fatalities were probably comparable in the rest of Europe, although records are less complete. Yes, 20%. Influenza was still a relatively new disease to Europeans at that time and was still dangerous. An outbreak of “coughing violence” was also recorded in Japan in 1556, and large numbers reportedly died. Then as now, influenza presumably came from China, where different virus strains from pigs, poultry, and people hybridized generating new variants. The flu epidemic of 1556 reached America in 1558 and so was the first true worldwide pandemic to achieve a major death toll in both the Old World and the New World.

  A variety of other Old World diseases, such as mumps and diphtheria, migrated to the New World over the next couple centuries. Smallpox and measles, the two biggest killers, broke out every so often. Relatively isolated tribes often survived unexposed for several centuries. The Mandan tribe of North America was reduced from several thousand to only 30–40 in 1837. They were besieged by the Sioux and crowded together, unable to get away, when an epidemic broke out. The Cayapo tribe of South America was safely isolated until a single missionary visited them in 1903. By 1927, the tribe had shrunk from about 7,000 to less than 30.

  The religious implications

  The colossal death toll among the indigenous inhabitants convinced the Christian invaders of North America that their occupation of the Americas was approved and foreordained by God. Conversely, the Aztecs and Incas felt that their own gods were angry and had disowned them in favor of the Europeans. Neither side understood the nature of disease, but the fact that the same diseases that decimated the American Indians caused so few casualties among the Europeans was decisive to both. The Aztecs and Incas were demoralized, and the survivors were easy prey to Catholic missionaries spreading Christianity. It made sense to worship the God of the victors.

  The Puritans who founded the Plymouth colony believed that God cleared away the Indians for their benefit. Only divine intervention could account for the incredible mortality among the Indians while leaving the colonists essentially unscathed. According to Puritan leader Cotton Mather, “the woods were almost cleared of these pernicious creatures, to make room for a better growth.” Perhaps it is hardly surprising that the Puritans regarded themselves as favored by God. In the days before science revealed microorganisms to human view, there was no other convincing explanation.

  One outstanding theological problem at that time was the origin of the indigenous Americans. If Adam and Eve had been created in the Garden of Eden, somewhere in the Middle East, how did America get its pre-Columbian population? Many Puritans believed that the devil had lured the Indians to America. Here in splendid isolation, the devil was free to rule over the Indians without worrying that the Gospel might intrude.

  When smallpox, measles, and other virulent viral diseases first appeared, they doubtless devastated Old World civilizations, much as America was devastated in the years following Columbus. The responses to the pestilence that swept the American continent yield some insight into the behavior of ancient Europeans and Asians, who faced similar catastrophes long before accurate records were kept. Both invaders and the invaded agreed that the massive death toll could have had only one cause: God’s will. The invaders were convinced that the Almighty had foreordained their occupation of America. When we find historical Old World cultures utterly convinced that God is on their side, we might well wonder if it is for the same reason. For instance, the Bible tells how the Angel of the Lord killed 185,000 Assyrians in one night, so relieving the siege of Jerusalem by Sennacharib the Assyrian.

  Deliberate use of germ warfare

  In 1763, during the French and Indian War, Lord Jeffrey Amherst ordered blankets contaminated with smallpox to be distributed among enemy tribes of Indians. Captain Ecuyer gave blankets from the smallpox hospital at Fort Pitt to two Indian chiefs. Although there was a severe outbreak of smallpox among the Indian tribes of the Ohio Valley, it is debatable whether the blankets caused it. Smallpox was already in the area, hence the local smallpox hospital from which the blankets came. Many other stories exist of items contaminated with smallpox or measles being given to North American Indians, and tales of settlers deliberately passing on these diseases are part of colonial folklore.

  Until the late nineteenth century, disease was thought to originate in dirt, sewage, refuse, and swamps, and the odors and vapors arising from them. Thus, in 1699, a German missionary commented, “The Indians die so easily that the bare look and smell of a Spaniard causes them to give up the ghost.” Many colonists may well have believed that contact with something soiled, such as blankets in which a smallpox victim had slept, was necessary to spread the disease. Having seen the devastating effects of smallpox or measles on Indians, they naturally concluded that the Indians had somehow obtained contaminated materials. So many of the tales are probably later attempts at rationalization more than historical observation.

  Today we know that measles and smallpox are transmitted from person to person through the air, in tiny droplets breathed out by infected victims. Although viruses can indeed survive for some time on inert objects before they infect another victim, this is not nearly as effective as direct droplet transmission. Once European diseases reached the American continent, they needed little help in transmitting themselves and often moved ahead of the European invaders. For example, in 1616–1617, a major epidemic devastated the Massachusetts Bay area. The Pilgrims did not land until three years later, by which time the indigenous population had already been thinned out.

  Slavery and African diseases

  One result of the massive die-off of the indigenous Americans was a shortage of cheap labor. The colonial powers responded by importing slaves from the Old World, who were consequently relatively resistant to Old World diseases. In practice, this meant enslaved Africans who brought a variety of tropical diseases with them, notably malaria and yellow fever. These two diseases caused havoc in the tropical parts of the New World and are still major problems today in parts of South America. Of course, these diseases were also current among the Europeans and would have reached America whether or not slavery was practiced.

  It is sometimes suggested that many human diseases originated in Africa. Doubtless some original human diseases (such as malaria, typhoid, and herpes) came from Africa because Africa is the home of the human species. However, until relatively recently, the population of sub-Saharan Africa was rather sparse.
It is unlikely that most Old World epidemic diseases originated in sub-Saharan Africa. The dense populations of Mesopotamia, the Indus Valley, the Yellow River Valley in China, and the Nile Valley are more likely sites of origin.

  Exposure of islands to mainland diseases

  The islands of the Pacific Ocean were isolated from the rest of Eurasia, and not surprisingly, the various European voyages of discovery had much the same effect on the natives of the Pacific islands as on the Native Americans. For example, in 1875, measles reached the island of Fiji. Between 25% and 30% of the population died. Because individual islands are scattered over vast expanses of ocean, the smaller and lonelier ones have often remained uninfected until recently. They were usually exposed to one new disease at a time, giving their populations time to recover between onslaughts.

  The islands of Britain and Japan were both relatively isolated from the neighboring continental mainland in early medieval times. Only after more efficient ships were developed and trading increased significantly did they reach infectious equilibrium with their respective mainlands. Consequently, England and Japan both experienced major die-offs due to epidemics brought from the disease-ridden mainland. Their population densities lagged well behind that of the mainland until relatively late in the Middle Ages. The Japanese population crossed the “epidemic threshold” and doubled between 1100 and 1300, whereas in England, this was delayed until after the Black Death of the mid-1300s.