Just as genetic variation and natural selection are key components of evolutionary medicine, models of equilibrium and change are central to medical ecology. Fluctuations among, or disruption of biotic, abiotic, and cultural subsystems are part of normal cycles and can be accommodated to a certain extent through a variety of adaptive mechanisms, both at the individual level and the population level. But when too severe an imbalance occurs, repercussions may include environmental degradation, loss of resources, population decline, changes in trophic (feeding) relations, and disease. This model suggests that the health of a population is a function of its ecosystem and of the adaptive mechanisms used by the population to maintain its place in the ecosystem (Moran, 2000). Yet adaptation does not always lead to optimal health; examples abound throughout history and prehistory of populations threatened by the long-term repercussions of their own subsistence, as well as of societal segments that do not benefit or flourish within the midst of profitable and productive economies.
When human activities (e.g., farming or building roads) disrupt the ecological niches of other fauna, subsistence changes may bring increased food security but also increased disease prevalence. On the one hand, the massive development projects of the last century have contributed to sharp increases in the incidence of previously endemic diseases. On the other hand, public health promotion of childhood immunization, nutritional supplements, improved water systems, and disease prevention have led slowly to the "epidemiological transition" of the 20th century, with lowered infant mortality and longer average life expectancy.
Infectious disease patterns vary by subsistence type and by region, but no society is free from disease. Medical ecologists are particularly concerned with "emerging" (and re-emerging) infectious diseases: HIV/AIDS, dengue fever, West Nile virus, antibiotic-resistant strains of tuberculosis, trypanosomiasis, schis-tosomiasis, Chagas' disease, and others. Malaria is among the most significant of the resurgent diseases. Once relatively contained by DDT, the anopheline vectors are now resistant to insecticides, and the parasites causing malaria have mutated into resistant strains. The current crisis in preventing malaria is due not only to biological change in disease agents, but also to poverty, malnutrition, and inadequate healthcare in the regions most sharply affected. This case demonstrates political ecology, an approach which includes economic and social factors in conceptual models (Brown, Inhorn, & Smith, 1996).
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