Using descriptions, personal stories and anecdotes, this study tells of the origins, biology and destiny of diseases such as bubonic plague, malaria, typhoid, syphilis and AIDS. Drawing on evidence from places as remote from each other as the slums of Calcutta and the Peruvian rain forests, the work shows how the evolution of plagues has also played an important role in shaping the human species. It emphasizes that plagues are unusual events and the cells which cause them have often gone out on a evolutionary limb, risking their own destruction. The scientific insights demonstrate how plague organisms risk wiping out the hosts that harbour them, thus making an epidemic a type of evolutionary suicide for the organism itself.