The continual expansion of human populations since prehistoric times, and particularly since the advent of settled agriculture with its associated domesticated animals exposing humans to their pathogens as well as those spilling over from wild animal populations, has incrementally added to the pathogen load through successive invasions by different organisms over time (49, 50) . Well established principles of population ecology, applied via mathematical epidemiology as Anderson and May and others so aptly have done, readily explain why, ceteris paribus, infectious disease incidence should generally be increasing with human population size, as it has in the world's poorest and most populous regions. Yet in spite of what are in general commonly understood principles, and warning signs that went unheeded in the 1970's and 1980's (10, 30), biomedical and public health institutions were unprepared for the recent surge in emerging infectious diseases (1, 9, 10, 51) . Not until the 1990's, prompted in part by the HIV/AIDS pandemic and the failure of 'quick-fix' solutions based on drugs, vaccines, and pesticides (18) , did the biomedical science and public health communities begin to launch a significant response.