Infectious diseases: an ecological perspective

Microbes have played a decisive role in human history. Between 1348 and 1352 in many European countries plague was estimated to have killed a third to a half of the population.1 2 It was not swords and guns but imported microbes, carried by explorers over oceans, that defeated native populations in the Americas,3 and in Australia and southern Africa the arrival of Europeans killed off local populations by introducing infectious diseases. Local flora and fauna were also irreversibly altered. Many of these fertile, temperate, and now less populated lands were subsequently settled by Europeans.3

By the middle of the 20th century, infectious diseases were no longer the major causes of mortality in developed countries. The eradication of smallpox reinforced the perception that infectious diseases could be eliminated. Improved sanitation, clean water, and better living conditions, along with vaccines and antimicrobial agents, brought many infectious diseases under control in industrialised countries, but infections continued to kill millions each year in the developing world. Infectious diseases remain the most common single cause of death in the world today. Of the 51 million deaths worldwide in 1993, an estimated 16.4 million resulted from infectious and parasitic diseases.4 In sub-Saharan Africa, communicable diseases account for more than 70% of the burden of ill health (as measured by disability adjusted life years), in contrast to about 10% in industrialised countries.5

Increasingly humans have changed the earth in ways that make it easier for microbes to move and to reach vulnerable populations. Widespread use of antimicrobial agents and chemicals produces selective pressure for the survival and persistence of more resistant populations of microbes, and also of more resilient insect vectors.6 Patterns of infectious diseases are changing globally and on a massive scale.7 8 Although poor and disadvantaged people are …