As we discussed in Chapter 1, we can apply the conventional levels of prevention – primary, secondary and tertiary – to the way we think about protecting the world from infectious diseases. The first, or primary, level is preventing a disease from emerging; the second level is preventing a local outbreak; and the third is preventing international transmission, or a pandemic. In the next three chapters, we will explore each of those levels in greater detail, beginning at the most basic level: the factors contributing to disease emergence. All of the factors linked to emergence are in essence created or caused by humans; some of them are more feasible for humans to correct than others. One element is the increasing burden on the planet, with its limited resources. Another is a new ecosystem of sorts that we have created through burgeoning trade and travel – a world of high mobility and no boundaries. Also factoring into the equation is intensified food production practices. And finally, we have a growing array of medical practices with unknown consequences, such as xenotransplantation and widespread antibiotic use. Figure 6.1 adds such mechanisms to our original prevention pyramid.