Plagues and Peoples

1. Disease has shaped human history, often more decisively than battles or politics.

In PLAGUES AND PEOPLES, a fascinating exercise in historical speculation, William H. McNeill argues convincingly for the extraordinary impact of disease on human history.

Disease as a historical force. McNeill challenges traditional historical narratives by placing infectious disease at the forefront. He argues that disease outbreaks have often been the deciding factor in major historical events, influencing the rise and fall of civilizations, military campaigns, and cultural shifts. This perspective shifts the focus from solely human agency to the interplay between human societies and the microbial world.

Examples of disease impact. The book highlights instances where disease altered the course of history:

• Smallpox epidemic during the Aztec's siege of Cortez's army.
• The Black Death's decimation of Europe's population.
• The role of disease in the Spanish conquest of the Americas.

A new lens for understanding history. By recognizing the profound impact of disease, we gain a more nuanced understanding of the past. Events previously attributed solely to political or military factors are re-examined through the lens of epidemiology, revealing the often-overlooked role of invisible pathogens.

2. Humanity's relationship with disease evolved with shifts in lifestyle, from hunter-gatherer to agricultural societies.

Looked at in this way, world history offered a number of parallels to what happened in the Americas in the sixteenth and seventeenth centuries.

Hunter-gatherer disease landscape. Early human ancestors likely lived in relative equilibrium with their environment, including a diverse array of parasites and pathogens. Low population densities and nomadic lifestyles limited the spread of infectious diseases.

The agricultural revolution and new diseases. The shift to settled agriculture brought about significant changes in human-disease interactions. Increased population density, close proximity to domesticated animals, and altered landscapes created new opportunities for disease transmission.

Disease as a consequence of progress. The development of agriculture, while enabling larger populations and more complex societies, also came with a cost: increased vulnerability to infectious diseases. This trade-off between progress and disease risk is a recurring theme in human history.

3. Civilization created new opportunities for disease transmission, leading to the emergence of "crowd diseases."

Infectious bacterial and viral diseases that pass directly from human to human with no intermediate host are therefore the diseases of civilization par excellence: the peculiar hallmark and epidemiological burden of cities and of countryside in contact with cities.

Civilization and disease. The rise of cities and civilizations created ideal conditions for the spread of infectious diseases. Dense populations, poor sanitation, and increased trade facilitated the transmission of pathogens.

"Crowd diseases" defined. Diseases like measles, smallpox, and influenza, which require large populations to sustain themselves, emerged as significant threats in civilized societies. These "crowd diseases" became a defining characteristic of urban life.

Animal origins of human diseases. Many of the infectious diseases that plague humanity originated in domesticated animals. Close contact with livestock allowed pathogens to jump species, adapting to human hosts and causing widespread illness.

4. The confluence of Eurasian disease pools had profound consequences, reshaping demographics and power dynamics.

Looked at in this way, world history offered a number of parallels to what happened in the Americas in the sixteenth and seventeenth centuries.

Distinct disease pools. By 500 B.C., different regions of Eurasia had developed their own unique sets of infectious diseases. These "disease pools" reflected local environmental conditions, agricultural practices, and patterns of trade and interaction.

Contact and exchange. As trade and communication networks expanded, these previously isolated disease pools began to merge. This confluence of diseases had significant consequences for human populations, leading to epidemics and demographic shifts.

Disease and power. Populations with greater exposure to a wider range of diseases often developed higher levels of immunity. This "disease experience" gave them a biological advantage over less exposed populations, influencing patterns of conquest and cultural exchange.

5. The Mongol Empire facilitated disease exchange across Eurasia, culminating in the Black Death.

The volume provides fascinating reading and emphasizes a perspective on events that is not often found in the treatment of history.

Mongol expansion and disease. The Mongol Empire, by connecting disparate regions of Eurasia, inadvertently facilitated the spread of infectious diseases. The movement of armies, traders, and travelers across vast distances created new opportunities for pathogens to jump between populations.

The Black Death's origins. The Black Death, one of the most devastating pandemics in human history, likely originated in Central Asia and spread westward along Mongol trade routes. The disease reached Europe in 1347, causing widespread death and social disruption.

Consequences of the Black Death. The Black Death had profound and lasting effects on European society, including labor shortages, economic upheaval, and religious and cultural shifts. The pandemic also altered the course of European history, contributing to the decline of feudalism and the rise of new social and economic systems.

6. Transoceanic exchanges after 1500 unleashed devastating epidemics in the Americas.

The lopsided impact of infectious disease upon Amerindian populations therefore offered a key to understanding the ease of the Spanish conquest of America-not only militarily, but culturally as well.

The Columbian Exchange. The arrival of Europeans in the Americas initiated a period of intense biological exchange. Along with plants, animals, and technologies, Europeans also brought a host of infectious diseases to which Amerindians had no immunity.

Demographic catastrophe. Diseases like smallpox, measles, and influenza decimated Amerindian populations, leading to a demographic collapse of unprecedented scale. The loss of life was so severe that it undermined native societies and facilitated European colonization.

Cultural and social disruption. The devastating impact of disease extended beyond mere mortality. Amerindian cultures and social structures were shattered, leading to widespread demoralization and the collapse of traditional belief systems.

7. Medical science and public health initiatives since 1700 have altered the ecological balance of disease, but new challenges persist.

Quite apart from details of what I have to say, everyone can surely agree that a 'fuller comprehension of humanity's ever-changing place in the balance of nature ought to be part of our understanding of history, and no one can doubt that the role of infectious diseases in the natural balance has been.

Advances in medical knowledge. The development of germ theory in the 19th century revolutionized our understanding of infectious diseases. This led to the development of vaccines, antibiotics, and other medical interventions that have dramatically reduced the burden of disease.

Public health initiatives. Public health measures, such as sanitation improvements, water purification, and vector control, have also played a crucial role in controlling infectious diseases. These initiatives have transformed urban environments and improved the health of populations worldwide.

Emerging challenges. Despite significant progress, infectious diseases remain a threat. New diseases continue to emerge, and old diseases are evolving resistance to antibiotics. Climate change, globalization, and human encroachment on natural habitats are creating new opportunities for disease transmission.

Last updated: April 20, 2025