How City Design Shaped the Spread of Disease in Europe and the Pre-Columbian Americas

Before Europe’s first contact with the Americas in 1492, civilizations of the New World were subjected to small waves of disease and infection from animals in regional environments or localized parasites. However, these ailments were usually centralized in small regions, and there is little evidence of them spreading throughout multiple civilizations. Europe, on the other hand, faced many large pandemics and epidemics such as the Black Death and the Great Plague of London. So what caused these civilizations to encounter diseases in such different manners?

As populations surged throughout the 14th to 16th centuries, infrastructure in cities such as London and Paris can keep packed around a centralized building or plaza to maximize limited space. The limited space for construction and the high population density of many European cities forced the narrowly spaced buildings upwards, further concentrating regions of sewage and waste. Even with improvements in sanitation over following centuries, conditions improved only gradually: even into the early 19th century, most Western cities “had no systematic street cleaning regime,” and the carcasses of animals were simply left to rot in the streets3. Europeans also lived alongside cattle, pigs, and poultry—the sources from which diseases like measles and influenza are thought to have jumped to humans—whereas the Americas had very few domesticated animals7. The inadequate cleaning protocols, combined with the dense narrow streets of cities, concentrated harmful bacteria and pathogens and reduced biodiversity among microbiomes as only certain ones could survive in the stagnant wastes.

The decline in biodiversity favored microorganisms that reproduced quickly, which were typically the ones that carried and spread infectious diseases. This dominance of harmful microorganisms led to a significant increase in the risk of disease exposure to humans and animals2. This consequential relationship is conversely modeled in the “dilution effect,” a still-debated ecological theory which holds that high biodiversity tends to inhibit the spread of infectious disease5.

It was a much different case in the isolated Pre-Columbian Americas where population density was generally lower than in Europe. Indigenous people built their cities in areas with ample natural resources such as rivers and jungles. Many Aztec and Mayan city layouts were constructed in precise geometric patterns influenced by the civilization’s religion, often including open plazas and spacious roads to support ceremonies and other spiritual events and rituals. Furthermore, unlike European civilizations, many of these American communities built their cities with nature rather than bending it to their will. The Inca Empire, for example, which lived on mountainous terrain, adapted to its surroundings by building temples and other infrastructure around nature, rather than renovating the land.

Due to the differing climates and natural environments, civilizations such as the Aztecs and Inca developed their cities on totally different bases, avoiding many of Europe’s problems even where populations were dense. Tenochtitlán, one of the most famous cities in the Americas, had more than 200,000 inhabitants by 1519, more than most European cities, yet it was clean and well planned, with aqueducts bringing in fresh water and a system for collecting trash1. Its orderly layout drew on a long tradition of urban planning in central Mexico: at Tula, the earlier Toltec capital that the Aztecs revered, archaeologist Michael E. Smith6 noted that “large buildings are balanced symmetrically across a square plaza” and that every building follows the same orthogonal grid, creating straight paths and sectors that helped with administration and management. Similarly, the Inca independently developed a different layout, using “kanchas,” rectangular and symmetrical walled enclosures adjoined with other similarly designed buildings. Both these examples have ample spacing in their layouts, which made it harder for microorganisms to become concentrated in areas throughout the city. Not only that, the open design and presence of aqueducts allowed plenty of sunlight and water to regularly interact with the land, supporting plant life and healthier soil, which in turn sustained a more diverse and stable microbial environment.

Compared to European designs, the municipal designs of the pre-colonial Americas maintained the balance in biodiversity across the land, disregarding compact streets for a more “unobstructed” design. This balance allowed for a variety of microorganisms to survive, keeping the chance of disease transmission among microbiomes to humans low. Ironically, the same isolation that spared these cities from Old World epidemics would leave their people with no immunity to European disease: when smallpox reached Tenochtitlán in 1520, it killed a large share of the city’s population, including its ruler, Cuitláhuac4.