Infectious Diseases and Human Population History

rom the plagues of biblical times to the HIV pandemic of today, infectious diseases have played an indisputably major role in human history. The continual expansion of human populations since prehistoric times has led to successive invasions of the human population by increasing numbers of different pathogens. Today many people's worries about emerging pathogens have been sharply focused by the Ebola virus outbreak in Kikwit, Zaire, and by Lyme disease and hantavirus outbreaks throughout the United States (Garrett 1994, Levins et al. 1994). In this article, we examine the infectious diseases of humans from an ecological perspective. Understanding pathogens at the population level is as important in disease prevention and control as understanding pathogens at the microscopic or molecular level. Three ecological processes are crucial in determining the impact, persistence, and spread of pathogens and parasites: the size and spatial distribution of the host population, the movement of infected and susceptible hosts and vectors, and the nutritional status of the human host population. Although medical ad-