ECOSYSTEM AND INFECTION DISEASES

The magnitude and direction of altered disease incidence due to ecosystem changes depend on the particular ecosystems, type of land use change, disease-specific transmission dynamics, and the susceptibility of human populations. Intact ecosystems play an important role in regulating the transmission of many infectious diseases. The reasons for the emergence or reemergence of some diseases are unknown, but the main biological mechanisms that have altered the incidence of many infectious diseases include altered habitat, leading to changes in the number of vector breeding sites or reservoir host distribution; niche invasions or interspecies host transfers; changes in biodiversity including loss of predator species and changes in host population density, human-induced genetic changes of disease vectors or pathogens such as mosquito resistance to pesticides or the emergence of antibioticresistant bacteria and environmental contamination of infectious disease agents. Human contact with natural ecosystems containing foci of infections increases the risk of human infections. Contact zones between systems are frequently sites for the transfer of pathogens and vectors, whenever indirect transmission occurs, to susceptible human populations such as urban-forest borders, malaria and yellow fever, and agricultural-forest boundaries, hemorrhagic fevers such as Congo-Crimean and hantavirus. The different types and subtypes of systems, natural, cultivated, and urban may contain a unique set of infectious diseases such as kala-azar or plague in drylands, dengue fever in urban systems, and cutaneous leishmaniasis in forest systems, but some major diseases are ubiquitous, occurring across many ecosystems such as malaria and yellow fever.