Diseases caused by infectious agents have profoundly affected both human history and biology. In demographic terms , infectious diseases-including both great epidemics, such as plague and smallpox, which have devastated human populations from ancient to modem times, and less dramatic, unnamed viral and bacterial infections causing high infant mortality-have likely claimed more lives than all wars, noninfectious diseases , and natural disasters taken together. In the face of such attack by microscopic invaders , human populations have been forced to adapt to infectious agents on the levels of both genes and culture. As agents of natural selection, infectious diseases have played a major role in the evolution of the human species . Infectious diseases have also been a prime mover in cultural transformation, as societies have responded to the social, economic, political, and psychological disruption engendered by acute epidemics (e.g. measles , influenza) and chronic, debilitating infectious diseases (e.g. malaria, schistosomiasis). Today, the global epidemic of acquired immune deficiency syndrome (AIDS) provides a salient example of the processes underlying infectious-disease-related cultural transformations. As many of the examples cited in this review illustrate, human groups have often unwittingly facilitated the spread of infectious diseases through culturally coded patterns of behavior or through changes in the crucial relationships among infectious disease agents, their human and animal hosts, and the environments in which the host-agent interaction takes place.