The Klondike gold rush, sparked in August 1896, brought tens of thousands of people from around the globe into the Yukon. Whalers had reached the Mackenzie Delta from the Pacific in the same decade. The demographic wave crested in 1900, the same year as a combined influenza and measles epidemic spread through Alaska into the Yukon, known as the Great Sickness. Oral histories and archival evidence show that it was not the absence of immunity but rather the synergistic effects of multiple pathogens that produced this devastating epidemic and its consequences.

The most widespread evidence for parasite infection in medieval Europe is for species spread by poor sanitation, such as whipworm, roundworm, and the protozoa Entamoeba histolytica and Giardia duodenalis, likely related to the common use of human faeces as a crop fertilizer. The prevalence of infection by intestinal helminths has been shown to be at least a quarter to a third of the medieval population, with a broad north–south divide in the dominant types of parasite. While species spread by poor sanitation were present throughout the continent, in northern Europe where eating raw, smoked, dried, or pickled fish was common, fish tapeworm resulted. The use of dogs by farmers put them at risk of infection by Echinococcus granulosus as shown by calcified hydatid cysts. Human fleas and body lice helped spread the Black Death pandemic from the fourteenth century onwards. Medical practitioners thought intestinal worms were formed due to an excess of phlegm (one of the four bodily humours), while ectoparasites were formed due to putrefying humours, sweat, and grime. Delousing combs were widely used to remove head lice, while the wealthy in Italy applied mercury ointments to their hair.

Despite major investment in sanitation infrastructure, intestinal parasites spread by faecal contamination of food and water were a particular problem everywhere in the Roman world. Similarly, ectoparasites such as lice and fleas were common despite the Roman enthusiasm for washing in communal bathhouses and the use of delousing combs. However, some parasites seem to be much more regional in their distribution, likely due to climate variations. Fish and Taenia tapeworms, spread by eating raw or undercooked fish, pork, or beef were more common in northern Europe than southern Europe, possibly due to the fact that the hot climate in the south made raw fish and meat go off faster than in the cooler north. In contrast, malaria seems to have been much more common in the Mediterranean region than in northern Europe, as the warm climate of the south created breeding sites for the Anophales mosquito, which transmitted the parasite. Roman period medical texts by Galen and other physicians showed awareness of a number of parasites and tried to explain them in the context of the humoral theory. Treatment involved trying to rebalance the humours in order to return the individual to health.

Chapter three, “Diffusion and Amplification,” discusses the long era in which pathogens and parasites were extended to new regions. As human communities became more complex, networks of trade expanded and became denser, allowing for the rapid, long-distance transmission of intestinal pathogens. Over the first millennium and a half of the Common Era, the disease pool of Eurasia and northern Africa became increasingly integrated. In the late fifteenth century, some Old World intestinal pathogens crossed the Atlantic and became established in the Americas. By the early nineteenth century, the integration had become global. Rapid urbanization in the industrializing North Atlantic states created a crisis of urban fecal pollution. In response, the first public health reform movements emerged. Beginning in the first half of the nineteenth century, cholera pandemics spread along global trade routes and infected all the inhabited continents. This provoked the first efforts at the international control of disease.