ZOONOSES AND THE ORIGINS OF OLD AND NEW WORLD VIRAL DISEASES: NEW PERSPECTIVES
LINDA M. VAN BLERKOM
Disease affects culture, and culture in turn affects disease. Epidemic diseases provoke social and medical response, while cultural behaviors influence the transmission and evolution of infectious diseases. Acquired immune deficiency syndrome (AIDS) illustrates this well. Medical procedures and sexual behavior have changed as a result of the AIDS epidemic, while cultural responses such as screening blood and using condoms have slowed its spread. Changing human behavior probably contributed to the origins of the AIDS pandemic ( Feldman 1990; Ewald 1994), and the AIDS virus has continued to evolve since then ( Yokoyama and Gojobori 1987; Li et al. 1988; Gao et al. 1992; Sternberg 1992). HIV and other emerging viruses are powerful reminders that we do not control our environment and cannot prevent threats to human health posed by new infectious diseases.
The appearance of new human infections is dependent on a number of factors that favor transmission and endemicity. Demographic factors such as population size, density, and interaction with other populations are important because causative agents need a minimum host population size in order to continually find new susceptibles. This size varies with the requirements and characteristics of each disease. Varicella zoster virus (chicken pox) remains latent throughout its host's life, often causing painful eruptions of shingles in old age. This virus requires only a few thousand people in contact for its persistence, while measles, with its short, virulent infection resulting in lifelong immunity or death, must have a host population of at least half a million ( Black 1966). More densely settled societies suffer more frequent epidemics, as do those with extensive trade and communication networks that include other large societies. The presence of several civilized centers in the Old World and their increased interaction contributed to the many