Infectious Disease

By living and interacting in densely populated settlements, human beings make it easier for disease-causing microorganisms to jump from one host to the next. Crowding, migration and easy travel dramatically increase the opportunities for the spread of infection. A growing population size expands the pool of humanity that parasites and other organisms can exploit. Writing about the growing risk of infectious disease Joshua Lederberg, who won a Nobel prize for his work in genetics, identified the "preponderant changes" behind the increased risk as "the sheer expansion of our species, with high population densities, and much the worse, egregiously stratified by standards of economics, nutrition, housing, and public health."1

A 1996 report by the World Health Organization noted the hazards of new settlements in formerly uninhabited countryside‹a phenomenon directly related to population growth‹because the process can expose human beings to previously unknown disease organisms, such as the human immunodeficiency virus (HIV) and the Ebola virus.2 The mounting use of antibiotics and other drugs for billions of episodes of disease each year contributes to the increasing microbial resistance to common drugs that is now hobbling disease control around the world. Even growth in the food supply, necessary to feed larger populations, can increase our vulnerability to disease. As Martin J. Blaser pointed out in an editorial in The New England Journal of Medicine, the potential for microbes to undergo their own population explosions is "implicit in large-scale food production," and because of this, "the opportunities for foodborne transmission of disease seem to be increasing."3

Although there has been little research on the direct links between population dynamics (size, growth and density) and disease outbreaks, some data suggest the likelihood of such links. A study of dengue hemorrhagic fever in Bangkok concluded that the mosquito that causes the debilitating tropical disease could establish itself only in urban areas that passed critical population thresholds. In Thailand between 1960 and 1972 there was a close correlation between the pace of urban population growth and reported cases of dengue.4

Added to the problems of population density and mobility are other factors related to population growth: People who are malnourished or lack safe sources of water and sanitation are vulnerable to illness. Global warming threatens to expand the range of tropical insects and other organisms that can spread disease. The increasing pressure to achieve high crop yields through pesticide use adds to the dangers of human exposure. Some of these chemicals are persistent organic pollutants now suspected of having long-term impacts on the reproductive systems of humans and animals. Because international trade carries foods around the world, while wind and water carry pesticide byproducts across borders, no one on the planet is beyond the reach of chemicals used legally or illegally in any country.

At the country and community level, governments often lack the resources or the will to keep sanitation and public health services growing as fast as population. At the household level, evidence from demographic surveys suggests that children born after several siblings tend to receive fewer immunizations and less medical attention for fevers and other illnesses than first born or second born children.5 The cumulative effect of all these influences is a greater risk of disease with higher birthrates and rapid population growth.