WHEN GERMS MADE HISTORY

Infectious diseases don't just make individuals sick - they can cause seismic shifts in societies. Jonathan Kennedy charts six moments when pathogens such as plague, smallpox and malaria played key roles in major cultural, political and economic transformations

- Jonathan Kennedy

The cultivation of disease

How infections that evolved alongside agriculture wreaked havoc on ancient empires

The adoption of settled agriculture was perhaps the most important turning point in history. For 2 million years, humans had been hunter-gatherers. Then, about 12,000 years ago, as the end of the last ice age brought a warmer and more stable climate, communities in various parts of the world began to cultivate crops and domesticate animals.

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. 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.