The human population of the earth took the whole of its existence until 1800 to build to 1 billion. By 2000 it had exceeded 6 billion, more than doubling in the twentieth century alone. In 1800, the time taken to navigate the globe by sailing ship was about a year. Today, no two cities served by commercial aircraft are more than a couple of days apart. Since this is less than most disease incubation times, infected people can travel undetected—a concern noted from the early days of commercial air travel. Within developed countries, the rate of individual circulation (in terms of average distances travelled) has increased 1,000-fold in the last 200 years. While the processes of population growth and geographical churn have been at work for the whole of human history, it is in the last two centuries that the momentum of change has gathered increasing pace. As described in Section 2.1, McMichael (2004) recognizes four separate stages. (i) Early human settlements from c.5,000 to c.10,000 years ago enabled enzootic pathogens to enter Homo sapiens populations. Some of these encounters led to the emergence of many of today’s textbook infections: influenza, tuberculosis, cholera, typhoid, smallpox, measles, malaria, and many others. (ii) Eurasian military and commercial contacts c.1,500 to c.3,000 years ago with swapping of dominant infections between the Mediterranean and Chinese civilizations. As described in Section 2.2, the plagues and pestilences of classical Greece and Rome date from this period. (iii) European exploration and imperialism from c.1500 with the transoceanic spread of often lethal infectious diseases. The impact on the Americas, on Australasia, and on remote island populations is well known; ships’ crews and passengers were the devastating vectors. (iv) The fourth great transition is today’s globalization, acting through demographic change and accelerating levels of contacts between the different parts of the world to facilitate disease emergence, re-emergence, and spatial transfer. Global warming, the destabilization of environments, the unparalleled movement of peoples rapidly across the globe through air transport, are all part of an evolving host–microbe relationship (cf. Section 1.3.1).
: The Meaning of the Twentieth Century: The Great Transition . Kenneth E. Boulding.
