The Role of Disease in Human History

Based on William McNeill, Plagues and Peoples (Anchor, 1998).

Smallpox, bubonic plague, malaria, tuberculosis: why do these deadly diseases and so many others exist? There are two possible explanations, one Biblical and one evolutionary. Mark Twain laid out the Biblical explanation in Letters from the Earth. Since the only living creatures that survived the Flood were those aboard the Ark, the microbes that cause these diseases must have been taken aboard along with the lions, elephants and giraffes. These microbes had to be “housed” in Noah and his family.

They were full of microbes. Full to the eyebrows; fat with them, obese with them, distended like balloons. It was a disagreeable condition, but it could not be helped, because enough microbes had to be saved to supply the future races of men with desolating diseases, and there were but eight persons on board to serve as hotels for them…There were typhoid germs, and cholera germs, and hydrophobia germs, and lockjaw germs, and consumption germs, and black-plague germs, and some hundreds of other aristocrats, specially precious creations, golden bearers of God’s love to man, blessed gifts of the infatuated Father to his children — all of which had to be sumptuously housed and richly entertained; these were located in the choicest places the interiors of the Family could furnish: in the lungs, in the heart, in the brain, in the kidneys, in the blood, in the guts. In the guts particularly. The great intestine was the favorite resort. There they gathered, by countless billions, and worked, and fed, and squirmed, and sang hymns of praise and thanksgiving; and at night when it was quiet you could hear the soft murmur of it.

Twain has covered the Biblical explanation pretty thoroughly, so I will confine myself to the evolutionary explanation: humans are an environmental niche, and organisms arise through mutation to populate it.


The survival of a colonizing organism depends upon its ability to adapt to the human body, and upon the body’s ability to adapt to it. An organism that kills its host too quickly is unable to propagate and becomes extinct. On the other hand, an organism that provokes very strong resistance within the host creates a crisis for both the host and the organism: either or both can be killed. A successful adaptation is one in which the organism survives without excessive harm to the host.

Successful adaptation has often occurred. We carry large populations of bacteria in our intestines without problem, and smaller populations on our skin and in our mouths. Many of these bacteria are actually beneficial to us.

Many virulent diseases are the consequence of unsuccessful or incomplete adaptation. Malaria is one example. The malaria plasmodium lives within human red blood cells. It periodically destroys millions of these cells. This destruction produces fever in the host, but leaves the plasmodium floating freely in the blood, where it can be taken up by a mosquito that bites the host. The plasmodium replicates within the mosquito, then moves to the mosquito’s salivary glands. It is injected into a new host with the mosquito’s next bite. The mosquitos must be active to play their role, so the plasmodium has adapted to do little harm to them. Weak humans do not interfere with the plasmodium’s life cycle, so there is no evolutionary pressure for the plasmodium to reduce the harm that it does to humans. Bubonic plague is another example. The microbe that causes it normally infects burrowing rodents and their fleas, and only occasionally infects humans. It has adapted to do little harm to the rodents and the fleas that it depends upon; but humans are so peripheral to its life cycle that there has been no evolutionary pressure to reduce harm to them.

The microbes that cause schistosomiasis, African sleeping sickness and typhus also have complex life cycles, and like the malaria plasmodium, they have adapted to their non-human hosts but not their human hosts. The microbes that cause tuberculosis, measles, smallpox, chicken pox, whooping cough, mumps and influenza have no non-human hosts. Their survival has depended on successfully adapting to their human hosts, and they have done so.

Microbes adapt to humans, and humans adapt to microbes. It is not humans as a whole that adapt, but only populations of humans. There are a number of diseases (including measles, smallpox, chicken pox, whooping cough and mumps) for which human adaptation means the ability to produce an antibody that fights off the disease. These diseases do little lasting harm in an adapted population; but in a population that has not adapted, a person’s first exposure does not trigger an antibody and is often fatal.

Adaptation takes time, so the first appearance of one of these diseases produces an epidemic with a high death toll. Subsequent recurrences of the disease are less and less harmful, as adaptation to the disease spreads through the population. This phenomenon was seen repeatedly in the history of the Americas. At the time of the Spanish conquest, the Incas and Aztecs were nearly annihilated by their first exposure to European diseases. European settlement of North America’s eastern seaboard was likewise accompanied by disease epidemics that are believed to have killed more than nine-tenths of the native population. As late as the mid-twentieth century the Inuit were killed in large numbers by their first exposure to diseases such as measles and tuberculosis.

Cities and Contagious Disease

Disease microbes that spread from person to person, either through the air or through physical contact, provoke the production of antibodies in adapted populations. Each person’s first exposure to the microbe makes him sick, but then the body produces the required antibody and fights off the infection. The body will “remember” how to produce this antibody for many years or even for a lifetime, so each person is effectively immune to the disease after his first exposure. The microbe’s survival (i.e., its transmission from one host to another) therefore depends upon the continued presence of people who have no previous exposure and therefore no immunity. These people are predominantly children, and the diseases produced by these microbes — such as measles, mumps and chicken pox — are often called “childhood diseases”. Today they are regarded as an inconvenience, but in earlier times when the diseases were more virulent and medicine was less advanced, they were often fatal. These diseases could not have existed before the rise of cities (about 3000 BC), because only in cities is the population of unexposed children large enough for disease to be readily transmitted.

The microbes for these diseases came into existence through the mutation of existing microbes. Settled agriculture accelerated this process by exposing humans to the microbes carried by their domesticated animals. The greater the variety of microbes in the environment, the greater the chance that one will mutate in a way that allows it to jump to the human population.

There were distinct microbe pools in four parts of Eurasia by 500 BC: the Mediterranean coast, Persia, the Ganges Valley, and China. The pools were distinct because people rarely travelled between these regions, so that a microbe that evolved in one region stayed in that region. Scientists can identify mumps, malaria, diphtheria, tuberculosis and influenza in the writings of Hippocrates (460–377 BC), but there is no sign of measles, smallpox or the bubonic plague.

The Americas appear to have been largely free of contagious disease when Europeans arrived. Genotype evidence has now shown that syphilis arose in the Americas and was carried back to Europe by Columbus’s sailors, but many diseases travelled in the other direction, with catastrophic effect. Why was the impact of disease so one-sided? There are two reasons. First, the people of the Americas became able to produce significant food surpluses only a few hundred years before the arrival of the Europeans. Food surpluses are a precondition for cities, and cities are a precondition for the appearance of contagious diseases. The late development of cities in the Americas meant that such diseases had little time to emerge before the appearance of the Europeans. Second, the domesticated animals kept by Europeans exposed them to a variety of microbes, some of which mutated and jumped to the human population. The people of the Americas had few domesticated animals: the most important ones were the llama and the alpaca of the Andes. They were therefore exposed to a much smaller variety of microbes, greatly reducing the chance that a mutation able to jump to humans would emerge.

The merging of the American and Eurasian microbe pools was devastating, but so had been the merging of the Eurasian microbe pools. These pools merged in the first and second centuries AD, when the Silk Road facilitated trade between Rome and China. Rome and China appear to have been the most seriously affected areas. Both were subject to repeated epidemics, some of which remained for more than a decade. In the West the major epidemics killed at least a quarter of the population in built-up areas. These epidemics included smallpox and measles, making their first appearances in human history. The Mediterranean population declined continuously for more than 500 years under the onslaught of contagious disease. In China the population fell from 60 million to 45 million over a period of 400 years.

Disease and Economic History

The primary effect of disease has been the pain, anguish and shortened lives of hundreds of millions of people. Against this backdrop it hardly seems appropriate to focus on its economic effects, but these effects were also substantial.

The population decline throughout Eurasia slowed both market development and technological progress. Large and dense populations permit a high degree of specialization and the high volume of trade that is necessarily associated with it. Adam Smith believed this kind of development to be the most important source of economic growth, and the epidemics that ravaged Eurasia set it back by centuries. There is also some evidence that technology progresses more quickly when more people are engaged with technological issues. If there are more people working on a particular problem, it is more likely that someone is going to have the insight that solves it.1 Also, technology often involves combining ideas in novel ways, so the more new ideas are being produced, the greater the likelihood that they will be combined in an interesting way.2 Disease, by decimating populations, reduced specialization and slowed technological progress.

The bubonic plague, which swept through Europe repeatedly in the fourteenth and fifteenth centuries, altered the course of its economy. Europe’s fundamental economic institution at that time was the manor, a self-sufficient agricultural establishment whose work was done by serfs. Each serf (and his children, and his children’s children) was tied to a particular manor, where he had some rights and many obligations. The manorial system wasted a great deal of the serf’s work potential. He was not free to choose the work he did. He could not move from a manor with excess labour to one where there was a labour shortage, nor could he go to a town and practice some entirely different trade. He had little incentive to work hard because the manor lord appropriated a large part of everything that he produced. Wasting labour in this fashion slowed Europe’s economic development. The first passage of the bubonic plague through Europe killed at least one-quarter of the population, creating a severe labour shortage on the manors. The serfs, recognizing the increased value of their labour, rebelled against the old system. In western Europe the manor lords were forced to make concessions, and the shift towards competitive markets for land and labour began. By 1500 most of the land was farmed by free labourers who were paid wages by the landowners, or by free farmers who paid rent to the landowners. Russia faced the same shortage of labour, but the serfs were not united and the lords were willing to violently suppress uprisings. The Russian serfs were reduced to virtual slavery, and remained in that condition until the Great Reforms of 1861. Western Europe had free markets for land and labour and its economies progressed; Russia wasted the talents of a large part of its population and was economically stagnant.3

The Silk Road gave Europeans direct access to China and its luxury goods (such as silk, porcelain, and spices). This route became much safer under the “Pax Mongolica” imposed by the Mongols in the aftermath of their conquests, and the volume of Europe’s trade with China rose. However, Europeans lost access to this route in the fourteenth century, in part because everyone feared the bubonic plague, and every stranger was regarded as a potential carrier. The closing of this route meant that Europeans were only able to acquire Chinese goods through the intermediation of the Arabs, who recognized their monopoly position and priced the goods accordingly. The Europeans’ desire to cut out the Arab middlemen was one of the factors (though not the main one) that led to the ocean exploration of the fifteenth century. This exploration would ultimately remake the world.

Lastly, as discussed above, European diseases devastated the aboriginal societies of the Americas. The destruction of these societies simplified the colonization of the Americas by the Europeans. This colonization is what made Latin America Latin, and what would ultimately make English the world’s most dominant language.

  1. The classic reference here is Michael Kremer, “Population Growth and Technological Change: One Million B.C. to 1990,” Quarterly Journal of Economics, 1993. Kremer argues that the melting of the ice at the end of the last ice age divided the world into five regions: Eurasia, the Americas, Australia, Tasmania, and Flinders Island. These regions had roughly equal technologies at that time. When they were reunited by the Age of Exploration, the sophistication of their technologies correlated exactly with their initial population: Eurasia at the top, followed by the Americas, Australia, and Tasmania, in that order. The society of Flinders Island had regressed technologically and then died out. The conclusion that he draws is that bigger populations have faster technological progress. However, the relative success of Eurasia and the Americas could also be the result of their ability to generate the food surpluses necessary for the rise of complex societies. Jared Diamond, in Guns, Germs, and Steel (Norton, 1997) argues that their ability to do so hinged not on their populations but on their natural endowments of domesticable plants and animals.
  2. James Burke’s Connections (Papermac, 1978) provides many historical examples of these linkages.
  3. See Douglass North and Robert Thomas, The Rise of the Western World (Cambridge, 1973).