Our themes have so far been commodity frontiers, the transformation of nature, the beginnings of imperial environmental regulation, and the spatial dimensions of colonial cities. In this chapter we open an interrelated enquiry—into disease, specifically the third pandemic of plague. The link between environments and disease has long been made. By the late nineteenth century, as parasitology and tropical medicine made rapid strides, the precise causes of life-threatening maladies could be identified with increasing accuracy. Ross pinpointed mosquitoes as bearers of malaria and yellow fever; Bruce and others discovered the trypanosome borne by tsetse flies (see Chapter 11). Such discoveries gave environmental investigation and understanding a new urgency. They also provided arguments and scientific rationales for environmental regulation that seemed to offer the possibility of controlling disease. State intervention in controlling disease had a profound impact on some colonial environments, both urban, in this case, and rural (Chapter 11). While environmental controls were conceived as beneficial, both for British and indigenous people, they were also one way in which imperial subjects experienced political and social domination. Medical management of environmentally related diseases was sometimes strongly contested by the colonized. The same dilemma is explored in relation to conservation (Chapter 16). In a seaborne empire, as we have noted, ports were often critical sites for urban development, and formed the fundamental web of early empire. Cities, especially ports, were also centres of disease. Infections were brought into them, both by ships and from their hinterlands. They provided large concentrations of people in which disease could easily take hold, especially in the absence of efficient sanitation and healthcare. More people were becoming urbanized, living on top of one another in unsanitary and overcrowded conditions that were ideal spawning grounds for new and old infections. By the 1890s, Mumbai, a major centre of infection in India, had about 800,000 people—along with Kolkata this was a new scale of ships ‘the most convenient way to travel’, and can easily scramble on board via mooring ropes. Other factors in its spread included the bigger size of ships (hence larger colonies of rats), the large shipments of grain (which provided food and nests), and the passing of infection from ships’ rats to susceptible populations of wild rodents in California, Argentina, and South Africa.