From exposure to disease: the role of environmental factors in susceptibility to and development of tuberculosis.

Throughout the first three quarters of the 20th century, the incidence of tuberculosis declined in industrialized countries (1,2) (figure 1). Part of this decline may have been due to isolation of infectious tuberculosis patients in sanitariums and the pasteurization of milk, but it is generally thought that improved housing and habitat, decreased crowding, better hygiene and sanitation, use of clean water, and better nutrition all contributed to decreased tuberculosis notification (3-5). Since the mid-1980s, however, this decreasing trend has slowed down and has even reversed in some countries, such as the United States and the United Kingdom, calling for rapid epidemiologic investigations (6, 7). In the United States, the resurgence of the disease in the 1980s was attributed to a group of factors, including the epidemic of human immunodeficiency virus (HIV) infection, diminished public health efforts to control tuberculosis, rising poverty, homelessness, overcrowded conditions, and immigration from countries with a high prevalence of tuberculosis (8, 9). Identification of these high-risk groups and behaviors has stimulated actions to improve tuberculosis control activities, resulting in a reversed trend in tuberculosis notification rates after 1994 (10, 11). However, developing countries never experienced such a substantial drop, and the number of reported tuberculosis cases increased dramatically during the 1980s, especially in Africa south of the Sahara, where tuberculosis is a leading cause of mortality (12). From 1985 to 1991, the annual number of reported new cases tripled in Zambia, doubled in Malawi, and increased by 76 percent in Tanzania (13) (figure 2). This increase in tuberculosis case rates in developing countries has been attributed mainly to the combined effects of HIV infection, population growth, and poorly organized tuberculosis control programs with low case finding and cure rates (13, 14).