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How Future Plagues Could Shape Human History: From the Rise of Christianity to the Fall of the Aztecs and the Importance of Public Sanitation.


“Every once in a while a book lands on your desk that changes the way you change the world you live in, a book that fundamentally challenges your understanding of human history.” Thus began the blurb that came with this book. Ah! I thought. The usual advertising hyperbole, a gross exaggeration.

Yet Pathogenesis did challenge much of my understanding of world history. Who knew Christianity would never have become a world religion had it not been for an Ebola-like pandemic in the 2nd century AD? Or that women without retroviruses would lay eggs instead of being born alive? (According to Kennedy, a retrovirus inserted DNA into our ancestors’ genomes that led to the development of the placenta.)

Book Review: Pathogenesis: How Germs Made History – by Jonathan Kennedy (Torva)

However, this is not another book of amazing facts: it is a scholarly work, with nearly 700 references and notes. At the same time it is very readable and sometimes even amusing.

Many books have been written about the impact of disease on civilization. I’ve even written my own records essay about the subject. However, pathogenesis delves deep into the social history of the world.

Jonathan Kennedy has a PhD in Sociology from the University of Cambridge and his sociological bent is very apparent. In eight chapters and some 350 pages, Kennedy takes us on a whirlwind journey through social history, describing how infectious diseases have shaped humanity at every stage.

Read more: Viruses are both the villains and heroes of life as we know it

‘It’s a bacterial world’

Kennedy begins by describing the three major branches of living organisms, bacteria, archaeaAnd eukaryotes – it is the latter containing all complex life forms, including humans. However, less than 0.001% of all species are eukaryotes.

Bacteria, on the other hand, are the dominant life form on this planet. As Kennedy puts it, “it’s a bacterial world, and we’re just crouching here.”

Our own kind, homo sapiens, originated about 315,000 years ago and lived for the most part in Africa. At the same time, human species such as Neanderthals and Denisovans spread to Europe. But about 50,000 years ago, homo sapiens erupted from Africa and spread around the world, while all other human species simply disappeared. There are many theories about why and how this happened – for example maybe homo sapiens were just smarter.

However, Kennedy proposes his own theory. Because homo sapiens lived mainly in Africa, were exposed to many pathogens and eventually acquired genetic changes that gave them some protection. The exodus from Africa exposed other species to these pathogens, causing them to die.

He describes the Neolithic Revolution, which took place some 12,000 years ago and saw the change from hunter-gatherers to farmers. Because of their nomadic existence in small groups, hunter-gatherers were generally relatively healthy, with an average lifespan of 72 years – better than the average lifespan in some countries today!

It has always been assumed that this revolution was a good thing, with better nutrition and more free time. However, according to Kennedy, the Neolithic Revolution led to the rise of despotism, inequality, poverty and backbreaking labour. He describes how settlement and pet-keeping led to the emergence of zoonotic diseases, that is, diseases spread by animals.

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Settlement and pet-keeping led to the rise of animal-borne diseases.
kallerna/Wikimedia Commons, CC DOOR

Read more: Disease evolution: our long history in fighting viruses

Plagues and social unrest

In a chapter on ancient plagues, Kennedy quotes from Monty Python’s The Life of Brian:

Okay, but aside from sanitation, medicine, education, wine, law and order, irrigation, roads, fresh water system, and public health, what have the Romans ever done for us?

He points out that Roman cities were in fact “filthy, stinking and disease-ridden”, and goes on to describe the great plagues that weakened the Roman Empire. The first was the Antonine Plague, possibly caused by smallpox. This was followed about 70 years later by the Plague of Cyprian of AD 249-262, which led to the split of the Roman Empire and the rise of Christianity.

Kennedy concludes this chapter by describing Justinian’s plague, caused by the bubonic plague. The mass deaths caused by this epidemic led to the downfall of the Roman Empire and the Muslim conquest of the Middle East.

In the period 1346-1353, the Black Death swept through North Africa and Europe, killing a estimated 75 million to 200 million people. Kennedy describes the devastation and massive social upheaval that resulted from this pandemic. Until then, the Roman Catholic Church dominated society. But:

During the Black Death and subsequent plague outbreaks, people looked to the church for comfort. But all too often they didn’t.

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The Black Death killed an estimated 75-200 million people in Europe and North Africa. Hugo Simberg Black Death.
Wikimedia Commons, CC DOOR

This led to the rise of Protestantism aided by the invention of the printing press – a labor shortage spurred the development of such labour-saving devices. Over the next 200 years, waves of plague repeatedly hit Europe. A quarantine system was developed in Venice and sanitary cord established, to prevent movement of people between cities – are bells ringing?

Read more: Did the Black Death spawn modern plagues?

Pathogens as Killers of the New World

In the period from 1500, white colonialists almost wiped out the native population by infecting them. Kennedy begins with the early 16th century, when Spanish conquistador Hernán Cortés led an expedition to Mexico. His arrival introduced smallpox, resulting in the total destruction of the Aztec Empire within just two years. However, this was just the beginning.

In the early 1530s, Mexico was hit by a measles epidemic that killed 80% of the population, making it the deadliest epidemic in recorded history. Over the following decades, the introduction of infectious diseases from Europe across the Americas led to a 90% drop in population.

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Hernán Cortés brought smallpox to Mexico, resulting in the total destruction of the Aztec Empire within two years, as illustrated in this 16th-century drawing of Aztec smallpox victims.
Wikimedia Commons

However, not only the New World was seriously affected by pathogens during this period. On the west coast of Africa, explorers and so-called colonialists died en masse from malaria and yellow fever.

Interestingly, Kennedy begins his chapter on revolutionary plagues with the assassination of George Floyd and the Black Lives Matter movement, before delving deep into the history of slavery. He describes slavery in Greek and Roman times and the thriving slave trade in the medieval Mediterranean.

The association between black Africans and slavery did not begin until the 15th century. Only 3% of the 12.5 million people trafficked across the Atlantic ended up in the United States. The most common destinations for the slave ships were the European colonies in the Caribbean, where African slave labor was first used more than a century before their shipment to North America.

Meanwhile, in the Caribbean, slave laborers from tropical West Africa toiled on sugar plantations owned by the English, Spanish, French and Dutch. Yellow fever transmitted by mosquitoes wiped out many Europeans, including military garrisons, leading to slave revolts.

Read more: Friday essay: a slave state – how blackbirding in colonial Australia created a legacy of racism

Diseases ‘thrived’ in Dickensian habitats

When Kennedy shifts his focus to Britain and the Industrial Revolution, he describes it as the change from a Thomas Hardy novel to a Charles Dickens novel. The overcrowded and unsanitary conditions in working-class urban neighborhoods created new habitats in which pathogens thrived.

Kennedy again calls upon Monty Python to evoke the landscape of the time and remind readers of the famous sketch of the four Yorkshiremen. The scene reminded me of another quote from the same skit:

You were lucky to have a house! We used to live in one room, all one hundred and twenty-six of us, with no furniture. Half the floor was missing; we all huddled in a corner for fear of falling!

Each Epidemiology 101 course covers the story of John Snow (no – not the “Winter is coming”!). Two decades before developing the microscope, Snow researched cholera outbreaks to discover the cause of disease and how to prevent it.

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John Snow proved in 1854 that cholera is a water-borne disease: a London pub was named after him.
ceridwen/Wikimedia Commons, CC DOOR

During the third British cholera outbreak in 1854, Snow famously removed London’s Broad Street water pump to demonstrate that cholera was a water-borne disease. There is one for those interested John Snow pub in London. Kennedy, of course, includes this story in his book.

Kennedy points out that 3.5 million people – half the world’s population – do not have access to proper toilets, while a billion do not have clean drinking water and that 1.5 million people, mostly children, die each year from waterborne diarrhea.

We still have massive outbreaks of cholera, especially in areas where normal life has been disrupted by war or natural disasters. Tuberculosis still kills 1.2 million people a year, despite the availability of antibiotics. Malaria kills another 600,000.

Finally, in this section he briefly addresses COVID. He points out that not everyone in the world has benefited from the medical advances brought about by COVID, and that the self-interest of high-income countries has robbed the poorer ones. As he puts it, “pathogens thrive on inequality and injustice”.

Read more: From fleas to flu to coronavirus: how ‘deathships’ spread disease through the ages

Future plagues

Kennedy concludes with a look at future plagues. He points to the precarious position of humanity: we live on a planet dominated by bacteria and viruses. He believes our best chance of surviving the threat of pathogens comes from working together and reducing inequality, both within and between countries.

Based on the title, I assumed this book would be about the role of pathogens in shaping civilization. Instead I found a social history of the world, with occasional forays into diseases and their impact on society. Nevertheless, I thoroughly enjoyed the book and can highly recommend it to people interested in history, sociology, and epidemiology.

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