6 moments when diseases drastically altered human history

The consequences of the adoption of farming are still debated. Many scholars see the transition to farming as the first crucial step on the path of human progress. Others point out that agriculture condemned the majority of the population to back-breaking, mind-numbing labour. The transformative impact of farming on infectious diseases is, though, abundantly clear.

• On the podcast | Kyle Harper discusses how infectious diseases have shaped our past

For the first time, humans lived in close proximity to livestock. This aided the emergence of zoonotic infections – diseases that jump from animals to humans. The crowded and insanitary living conditions in Neolithic settlements also encouraged the transmission of pathogens from person to person or via infected water. Then, as trade links between far-flung locations developed, those too helped epidemics spread.

Unsurprisingly, many of the most notorious infectious diseases – including the plague, tuberculosis, polio, smallpox and measles – emerged in the wake of the adoption of settled agriculture.

Infectious diseases played a crucial role in the rise and fall of the great empires of antiquity. In the fifth century BC, the Greek historian Thucydides argued that the Plague of Athens (possibly typhus or smallpox) was a crucial turning point in the Peloponnesian War, because it weakened Athens but left Sparta untouched. More recently, the American classicist Kyle Harper highlighted the impact of epidemics on the Roman empire.

• Read more | The reign of Roman disease: why did disease hit a flourishing empire?

The Antonine Plague (probably smallpox) of the mid-second century AD contributed to the end of the remarkable period of peace and prosperity known as the Pax Romana. The Cyprianic Plague (an Ebola-like virus) played a prominent role in the ‘Crisis of the Third Century’ – five decades of insurrection and political instability, from AD 235 to 284, when the empire nearly collapsed. And the Plague of Justinian (Yersinia pestis plague) halted the eastern empire’s efforts to reconquer the western provinces in the sixth century AD.

Black Death: the plague that restructured western Europe’s social systems

Until the arrival of one of the most horrifying pandemics in history, the standard of living across much of western Europe had not improved markedly for centuries. The decline of the Roman empire had created a power vacuum filled by warring feudal states. Kings granted large tracts of land to allies in return for loyalty. These aristocratic clans then entered into similar relationships with vassals.

At the bottom of the pyramid, lesser lords provided serfs with land to farm in return for a share of crops, free labour and military service. The feudal system discouraged innovation. Lords spent any surplus on building castles and armies to defend themselves or attack others. Serfs grew a variety of crops on strips of land in different fields to minimise the risk that their whole harvest would be destroyed by disease, animals or extreme weather.

• On the podcast | Jane Whittle looks at how the Black Death reshaped the economy and society of England

Their standard of living had not improved since Roman days. Then, in the mid-14th century, the Black Death struck. Historian Ole Benedictow estimates that plague killed over half of Europe’s population in five years. If just one pandemic hit, societies would have probably rebounded within a few generations. But the plague struck again and again over the next couple of centuries.

In England, the population did not return to its pre-Black Death level until the 18th century. This demographic collapse plunged the feudal system into crisis. Serfs tried to take advantage of the new scarcity of agricultural labour to negotiate better terms. Lords resisted, because the marked fall in the number of serfs had already put a massive dent in their incomes.

If just one pandemic hit, societies would have probably rebounded within a few generations. But the plague struck again and again over the next couple of centuries

Parliament passed laws that tried to maintain serfs’ obligations as they had been before the Black Death. This caused widespread anger, which boiled over in the 1381 Peasants’ Revolt. The lords eventually gave in to pressure from below, and offered serfs improved conditions. By the mid-15th century, most English peasants had won their freedom.

Over time, as American economic historian Robert Brenner points out, a new system emerged in which lords rented out their land at market-determined rates. The majority of rural inhabitants were made landless, but the most entrepreneurial peasants became commercial farmers, adopting the latest technology and growing the most lucrative crops. Innovation and specialisation led to astonishing growth in output. This transformed society.

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Farming was not only able to feed the rapidly growing urban population, but food became so plentiful and cheap that people had extra money to spend on things such as textiles and sugar, which in turn encouraged industrialisation at home and colonisation abroad.

The New World: conquistadors inadvertently export the pox to the Americas

In 1521, Hernán Cortés and his army of about a thousand Spaniards achieved one of the most remarkable feats of conquest in history. They captured Tenochtitlan, the Aztec city and home to a quarter of a million people – four times the size of Seville, then the biggest city in Spain – that ruled over a vast empire stretching from the Atlantic to the Pacific. Then, in 1532, Francisco Pizarro managed something even more implausible: he conquered the Inca, the largest and most advanced empire in the Americas, with a force comprising just 106 foot soldiers and 62 horsemen.

• Read more | Cortés and Montezuma: the conquering of Tenochtitlan

These victories were just the beginning. The Spanish ruled much of South and Central America for the next 300 years. Their language, religion and culture still dominate the region today. The Spanish conquest of the Americas was possible only because they were unwittingly aided by infectious diseases. After Europeans started settling in the New World, it was just a matter of time before pathogens from the Old World made the jump across the Atlantic.

Smallpox hit Tenochtitlan in 1520, the year after Cortés first landed in Mexico, killing between one-third and half of the population in the region in just a few months. The Spanish, who would have been exposed to the virus in childhood, were unaffected. Without this seemingly miraculous intervention, Cortés would never have defeated the Mexica.

Smallpox raced ahead of the conquistadors. In 1524, it killed a similar proportion of the Inca population – including the emperor, his designated heir and most of the nobility. A war of succession then further weakened the empire. This death and disruption made Pizarro’s victory possible.

• Read more | Francisco Pizarro's tiny Spanish army takes a mighty Incan leader hostage

Smallpox was just the beginning. The Americas were hit by wave after wave of infectious diseases, resulting in a 90 per cent fall in the population, from about 60.5 million in 1500 to 6 million a century later. The decline in slash and burn agriculture that followed this fall in population resulted in a marked fall in atmospheric CO2 levels, and the global surface air temperature cooled by 0.15°C.

Colonisation of Africa: malaria slows the European advance

Portuguese would-be conquistadors coveted west Africa’s natural resources – especially the gold made famous by the fabulously wealthy Malian ruler Mansa Musa’s pilgrimage to Mecca in the early 14th century. But, just as in the Americas, powerful states stood in their way. In contrast to the New World, the Portuguese made only minor inroads in west Africa, and their presence was restricted to isolated coastal feitorias (trading posts).

Part of the reason was that west Africa was linked to Europe and Asia by trans-Saharan trade routes – the same ones along which Musa travelled – so its inhabitants developed immunity to diseases such as smallpox. Another crucial factor was the deadly strain of falciparum malaria that thrived in the warm, humid climate of west Africa. It killed – and still kills – large numbers of children.

• On the podcast | Toby Green explores the history of West Africa and its relations with the wider world

Those who don’t die in infancy develop resistance, so falciparum malaria is a relatively harmless condition in adults who have grown up in the region. But Europeans bitten by an infected mosquito for the first time were very likely to die. American historian Philip D Curtin estimated that, in the late 18th century, between 30 and 70 per cent of Europeans died in their first year on the west African coast, with the majority of these deaths caused by malaria.

Little wonder that the region became known as ‘the white man’s grave’. The interior of the African continent was even more deadly, with the average European surviving for just four months in Mali. In effect, malaria created a defensive force-field that made military conquest of Africa by Europeans all but impossible. As late as 1870, only one-tenth of the African landmass was controlled by European states.

In the late 18th century, between 30 and 70 per cent of Europeans died in their first year on the west African coast, with the majority of these deaths caused by malaria. Little wonder that the region became known as ‘the white man’s grave’

These colonies were in temperate, less disease-affected regions in the far north (Algeria) or south (Cape Colony, in what’s now South Africa). It was the increased use of quinine to prevent and treat malaria (reducing European mortality in tropical Africa by about half) that made the ‘Scramble for Africa’ possible. By the turn of the 20th century, over nine-tenths of the continent had been colonised.

Slavery: yellow fever in the New World fuels the transatlantic slave trade

Slavery has existed for thousands of years, seen in ancient Greece and ancient Rome, but its association with skin colour emerged only in the past few centuries. When the Spanish arrived in the Caribbean, they realised that the warm, humid climate and volcanic soil were ideal for growing sugar cane. But the conquistadors needed to find workers to cultivate, harvest and process the crop – and indigenous populations were devastated by infectious diseases.

For centuries, Spain had used as slave labour Muslims captured during the so-called reconquest of al Andalus. After the fall of the Emirate of Granada in 1492, merchants turned instead to west Africa – a move that inadvertently set the American tropics on the path toward racialised slavery.

• Read more | How slavery thrived in Renaissance Europe

The Aedes aegypti mosquito, and the yellow fever virus it carries, hitched a ride across the Atlantic on slave ships. Caribbean sugar-cane plantations provided ideal conditions for the mosquito to reproduce. Yellow fever wasn’t a problem for enslaved west Africans who had been infected in childhood and acquired lifelong immunity, but European adults died in such high numbers that it was not viable to use them as labourers.

In the 1620s, the English established the colony of Barbados to cash in on the demand for sugar back home. Plantation owners initially employed indentured servants from Britain and Ireland to work on their plantations. After all, the agricultural revolution had created a glut of landless labourers looking for gainful employment. But after a yellow fever epidemic killed half of the island’s population in the mid-17th century, the plantation owners turned to enslaved Africans instead.

• Read more | Slavery's painful legacy: the British empire's role in the trade of enslaved people

Something similar happened in Britain’s North American colonies. Initially, the majority of labourers were indentured servants from Europe. In 1680, black Americans accounted for less than 5 per cent of the population. It was only after falciparum malaria made its way to the colonies in the mid-1680s that enslaved people from west Africa, who had developed immunity to that strain, began to form the majority of labourers.

By 1750, black Americans accounted for 61 per cent of South Carolina’s population, and 44 per cent in Virginia. In the northern colonies, where the climate was too cold for malaria, employers continued to prefer free Europeans over enslaved Africans. But more than 12 million Africans were trafficked in the transatlantic slave trade, and their descendants still suffer greatly from the pseudoscientific racist ideology developed to justify this iniquitous system.

Industrial revolution: rapid urbanisation wreaks havoc on public health

The development of the steam engine in the 18th century, thanks to James Watt, allowed humans to utilise the power of fossil fuels on an industrial scale for the first time. As industrialisation transformed manufacturing, Britain’s annual economic growth reached an unprecedented 2.5 per cent in the first half of the 19th century, before falling back to 2 per cent. In a world where economic growth had been close to zero since the adoption of agriculture, the impact was extraordinary.

• On the podcast | Emma Griffin tackles questions about Britain’s industrial revolution

People flooded into the towns from the countryside to work in factories, but for most of the 19th century at least, British politicians – who were elected by a small number of wealthy voters – weren’t interested in improving water and sanitation in urban slums. Such crowded and insanitary conditions created new habitats in which pathogens thrived.

In the mid-19th century, infectious diseases accounted for about 60 per cent of deaths in parts of Liverpool and Manchester. Diarrhoeal diseases, transmitted through drinking water contaminated with faeces, were the major killers. Between the 1820s and 1870 – a time of unprecedented technological development and wealth creation – average life expectancy in Britain remained stagnant at around 41 years.

Crowded and insanitary conditions created new habitats in which pathogens thrived

National figures were dragged down by the new industrial towns. In the central areas of Manchester and Liverpool, life expectancy during that period was around 25 years – lower than at any time since the Black Death. For factory labourers in Manchester and Liverpool, it was just 17 and 15 years, respectively.

Health in towns and cities began to improve only after political reforms in the late 1860s. Suddenly, more than 60 per cent of working-class men could vote in urban local elections. The new voters were far more receptive to city leaders’ ambitious plans to build vast, expensive water and sewerage infrastructure. These projects transformed public health, and deaths from water-borne infectious diseases began to fall.

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In the 1870s, life expectancy in Britain’s towns and cities finally rose above 1820s levels, and kept rising. It is only in the past decade that life expectancy has stopped increasing. It has even fallen since 2020, at least partly as a result of Covid-19. But it is too soon to say with any certainty how the most recent pandemic might transform our society, economics and politics.

This article was first published in the June 2023 issue of BBC History Magazine