Pathogenesis

Jonathan Kennedy argues that bacteria and viruses have been the protagonists in the most important social, political, and economic transformations in human history. Drawing on advances in ancient DNA analysis, genetics, archeology, and epidemiology, Kennedy contends that microbes, not "Great Men" or mass movements, have driven the emergence of the modern world. He traces this argument across eight thematic chapters, each organized around a major historical transformation.

Kennedy opens by establishing the scientific foundation for his thesis. He extends Sigmund Freud's observation that successive scientific revolutions have diminished humanity's sense of its own importance, arguing that the discovery of the microbial world represents another blow to human self-regard. Bacteria vastly outnumber all other life on Earth, and cyanobacteria made the planet habitable by producing atmospheric oxygen roughly 2.5 billion years ago. Viruses have inserted DNA into ancestral genomes, enabling functions essential to human existence, including the placenta that makes live birth possible. The human body hosts approximately 40 trillion bacteria, and gut microbes produce neurotransmitters that influence mood and behavior. Kennedy positions his approach as "history from deep below" (13), contrasting it with both the "Great Men" theory associated with Thomas Carlyle and the "history from below" championed by historians like E. P. Thompson.

The first chapter challenges the widely accepted explanation that Homo sapiens triumphed over other human species because of superior intelligence. Kennedy presents extensive evidence that Neanderthals were cognitively comparable: they manufactured sophisticated tools, used medicinal plants, buried their dead, cared for disabled community members, and created the oldest known cave art in Spain at least 65,000 years ago. Instead, he argues that infectious diseases explain why Homo sapiens replaced all other human species between 50,000 and 40,000 years ago. Africa's tropical biodiversity meant Homo sapiens carried a far greater disease load than Neanderthals in cold, sparsely populated Europe. With larger, more genetically diverse populations, Homo sapiens developed resistance to Neanderthal pathogens faster than the reverse, eventually encountering communities with no defense against African diseases.

The second chapter traces the epidemiological consequences of the Neolithic Revolution, the transition to settled agriculture that began roughly 12,000 years ago. Farming supported larger populations but also created what anthropologist James Scott calls "multispecies resettlement camps," enabling new zoonotic diseases to emerge as humans lived alongside domesticated animals for the first time. Kennedy uses ancient DNA to show that prehistoric Europe experienced two massive population replacements: first, olive-skinned farmers from Anatolia nearly completely replaced the indigenous dark-skinned Western Hunter-gatherers; then, fair-skinned Steppe Herders swept across Europe roughly 5,000 years ago, replacing the farming population that had built Stonehenge. A "Neolithic Black Death" of pneumonic plague may have caused population crashes of up to 60 percent in northwestern Europe. Kennedy concludes that modern Europeans are a genetic mixture of three distinct populations and that the Steppe Herders were the most likely source of all Indo-European languages.

The third chapter argues that pandemics destroyed the Roman Empire's power and facilitated the rise of Christianity and Islam. The Antonine Plague (165 CE onward), most likely smallpox, killed an estimated 7 to 8 million people and ended the pax Romana, a roughly 150-year period of stability and prosperity. The Plague of Cyprian (249–264 CE), likely a viral hemorrhagic fever, caused the empire to fragment. Kennedy draws on sociologist Rodney Stark's argument that Christianity grew explosively during these crises because it commanded believers to care for the sick, reducing mortality through basic nursing. The Plague of Justinian (541 CE), confirmed as bubonic plague through DNA analysis, devastated the Eastern Roman Empire and weakened both the Byzantines and the Sasanian Persians. Within decades of Muhammad's death in 632, Arab armies, largely protected from plague by their nomadic lifestyle, conquered vast territories from both empires, creating conditions in which Islam rapidly became a major world religion.

The fourth chapter traces the Black Death's role in transforming medieval Europe. The plague re-entered Europe via the Black Sea in 1347 and killed roughly 60 percent of the population. Kennedy describes the wave of anti-Jewish violence that accompanied the pandemic and argues that the plague's transformative power came from its recurrent nature, preventing demographic recovery for centuries. The differential impact on settled versus nomadic populations facilitated the rise of the Ottoman Empire. The Black Death also undermined the Catholic Church, prompting challenges from John Wycliffe, a 14th-century Oxford theologian, and later Martin Luther, whose ideas spread through the printing press. Most consequentially, Kennedy argues, the demographic crash triggered the transition from feudalism to capitalism in England: Labor scarcity dissolved serfdom and created a system of market-rate land rental that rewarded only profit-maximizing farmers, turning the majority into landless wage laborers.

The fifth chapter argues that Old World pathogens, not military superiority, made European colonization of the Americas possible. Smallpox reversed Hernán Cortés's initial defeat during La Noche Triste (1520) by killing between a third and half of the Central American population. Successive epidemics, including measles and cocoliztli, a salmonella-like infection, reduced the indigenous population of Mesoamerica from 20 million to 1.5 million within a century. Across the Americas, the population fell 90 percent. Kennedy contrasts this with Africa, where falciparum malaria and yellow fever made the continent a "white man's grave." Quinine eventually reduced European mortality enough to enable the Scramble for Africa in the 1880s, but continued disease risk shaped the extractive character of African colonialism, exemplified by King Leopold II's brutal Congo Free State.

The sixth chapter examines how malaria and yellow fever shaped racialized slavery. The early transatlantic slave trade inadvertently imported mosquitoes and parasites that made the Caribbean deadly for Europeans, driving plantation owners to replace European indentured laborers with enslaved West Africans who had immunity from childhood exposure. Kennedy draws on economist Elena Esposito's research showing that falciparum malaria appeared in the southern North American colonies in the mid-1680s, precisely when the enslaved population began to surge. The Mason-Dixon Line roughly corresponded to the boundary of the malaria parasite's range, driving the divergence between a slave-based South and a free-labor North. Malaria incapacitated over half of British troops at Yorktown, contributing to the American Revolution's outcome, and in Haiti, enslaved rebels used guerrilla tactics until yellow fever devastated Napoleon's expeditionary force, leading to Haitian independence in 1804 and Napoleon's sale of Louisiana to the United States. Kennedy argues that malaria's role in entrenching slavery also contributed to the polarization that led to the Civil War.

The seventh chapter argues that the Industrial Revolution initially produced devastating urban disease: Life expectancy for factory laborers in Manchester and Liverpool fell to 17 and 15 years respectively. Cholera, spread through contaminated water, became the emblematic epidemic. Kennedy traces how the newly enfranchised business class blocked investment in sanitation, and how the Second Reform Act of 1867, which enfranchised working-class men, broke this deadlock. Mayors like Joseph Chamberlain in Birmingham pioneered municipal infrastructure investment, and urban death rates tumbled.

The final chapter argues that preventable infectious diseases continue to kill millions in low-income countries while non-communicable diseases driven by poverty devastate rich nations. Kennedy presents China as the most notable example of escaping the "poverty trap," a concept from economist Jeffrey Sachs describing how disease and poverty reinforce each other: Life expectancy rose from 32 years in 1949 to 66.8 by 1980 through mass immunization and the barefoot doctor system, a network of community health workers serving rural populations. He argues that high-income countries "kicked away the ladder" by imposing Structural Adjustment Programs that forced developing nations to cut health budgets. Within wealthy nations, he documents stark inequalities, including a 27-year gap in male life expectancy between London's Kensington and Chelsea and the town of Blackpool, where local doctors coined the term "Shit Life Syndrome" (219) to describe their patients' common denominator of destitution and hopelessness. The Covid-19 pandemic exposed these inequalities, with the poor dying at far higher rates and fewer than 5 percent of sub-Saharan Africans vaccinated by the end of 2021 while rich nations stockpiled surplus doses.

Kennedy concludes that Covid-19 is the latest in a long series of disease-driven inflection points. He warns of emerging threats, particularly antimicrobial resistance (AMR), and argues that the great health improvements of the past resulted not from medicine or economic growth alone but from political decisions to invest in sanitation, housing, and poverty reduction.