The global scale, interconnectedness, and economic intensity of contemporary human activity are historically unprecedented,¹ as are many of the consequent environmental and social changes. These global changes fundamentally influence patterns of human health, international health care, and public health activities.² They constitute a syndrome, not a set of separate changes, that reflects the interrelated pressures, stresses, and tensions arising from an overly large world population, the pervasive and increasingly systemic environmental impact of many economic activities, urbanization, the spread of consumerism, and the widening gap between rich and poor both within and between countries.

Fifty years ago, the age-old scourge of infectious disease was receding in the developed world in response to improved public health measures, while the advent of antibiotics, better vaccines, insecticides and improved surveillance held the promise of eradicating residual problems. By the late twentieth century, however, an increase in the emergence and re-emergence of infectious diseases was evident in many parts of the world. This upturn looms as the fourth major transition in human–microbe relationships since the advent of agriculture around 10,000 years ago. About 30 new diseases have been identified, including Legionnaires' disease, human immunodeficiency virus (HIV)/acquired immune deficiency syndrome (AIDS), hepatitis C, bovine spongiform encephalopathy (BSE)/variant Creutzfeldt-Jakob disease (vCJD), Nipah virus, several viral hemorrhagic fevers and, most recently, severe acute respiratory syndrome (SARS) and avian influenza. The emergence of these diseases, and resurgence of old ones like tuberculosis and cholera, reflects various changes in human ecology: rural-to-urban migration resulting in high-density peri-urban slums; increasing long-distance mobility and trade; the social disruption of war and conflict; changes in personal behavior; and, increasingly, human-induced global changes, including widespread forest clearance and climate change. Political ignorance, denial and obduracy (as with HIV/AIDS) further compound the risks. The use and misuse of medical technology also pose risks, such as drug-resistant microbes and contaminated equipment or biological medicines. A better understanding of the evolving social dynamics of emerging infectious diseases ought to help us to anticipate and hopefully ameliorate current and future risks.

During the processes of human population dispersal around the world over the past 50 000–100 000 years, along with associated cultural evolution and inter–population contact and conflict, there have been several major transitions in the relationships of Homo sapiens with the natural world, animate and inanimate. Each of these transitions has resulted in the emergence of new or unfamiliar infectious diseases. The three great historical transitions since the initial advent of agriculture and livestock herding, from ca . 10 000 years ago, occurred when: (i) early agrarian–based settlements enabled sylvatic enzootic microbes to make contact with Homo sapiens ; (ii) early Eurasian civilizations (such as the Greek and Roman empires, China and south Asia) came into military and commercial contact, ca . 3000–2000 years ago, swapping their dominant infections; and (iii) European expansionism, over the past five centuries, caused the transoceanic spread of often lethal infectious diseases. This latter transition is best known in relation to the conquest of the Americas by Spanish conquistadores , when the inadvertent spread of measles, smallpox and influenza devastated the Amerindian populations. Today, we are living through the fourth of these great transitional periods. The contemporary spread and increased lability of various infectious diseases, new and old, reflect the combined and increasingly widespread impacts of demographic, environmental, behavioural, technological and other rapid changes in human ecology. Modern clinical medicine has, via blood transfusion, organ transplantation, and the use of hypodermic syringes, created new opportunities for microbes. These have contributed to the rising iatrogenic problems of hepatitis C, HIV/AIDS and several other viral infections. Meanwhile, the injudicious use of antibiotics has been a rare instance of human action actually increasing ‘biodiversity’. Another aspect of this fourth transition is that modern hyper–hygienic living restricts microbial exposure in early life. This, in the 1950s, may have contributed to an epidemic of more serious, disabling, poliomyelitis, affecting older children than those affected in earlier, more endemic decades. As with previous human–microbe transitions, a new equilibrial state may lie ahead. However, it certainly will not entail a world free of infectious diseases. Any mature, sustainable, human ecology must come to terms with both the need for, and the needs of, the microbial species that help to make up the interdependent system of life on Earth. Humans and microbes are not ‘at war’; rather, both parties are engaged in amoral, self–interested, coevolutionary struggle. We need to understand better, and therefore anticipate, the dynamics of that process.