Urbanization and infectious diseases: general principles, historical perspectives, and contemporary challenges

In 2009, a major demographic line was crossed: for the first time in history, the majority of the world population lived in cities rather than in towns and countryside (Fig. 4.1). This shift has been occurring over the past 100 years, with the most rapid rate of urban growth occurring over in the latter half of the twentieth century. Urban centers in the more developed regions of the world (i.e., North America, Australia, New Zealand, and Europe) experienced earlier growth in the 1920s1950s, and since then, the rapid rate of urban growth has been concentrated in the cities and towns of developing nations [1].