Understanding the mechanisms that underlie newly emerging and reemerging infectious diseases (EID) is one of the most difficult scientific problems facing society today. EIDs are diseases that have recently increased in incidence or in geographic or host range (e.g., tuberculosis, cholera, malaria, dengue fever, Japanese encephalitis, West Nile fever, and yellow fever), diseases caused by new variants assigned to known pathogens (e.g., HIV, new strains of influenza virus, SARS, drug resistant strains of bacteria, Nipah virus, Ebola virus, hantavirus pulmonary syndrome, and avian influenza virus), and bacteria newly resistant to antibiotics, notably the multiple resistant strains that render the armamentarium of antibiotics useless.
Fundamental questions persist concerning molecular mechanisms and specific cellular processes involved in pathogenesis, as well as transmission dynamics and epidemiology, of pathogens that cause some of the most studied of the reemerging infectious diseases, such as tuberculosis, malaria, and cholera. Newly emerging diseases caused by entirely novel or previously unrecognized pathogens, such as HIV/AIDS, SARS, and hantavirus, or those whose modes of transmission are currently under study, as in the case of Ebola and Nipah, represent yet another significant challenge. Certainly the mechanisms or processes of disease emergence involve factors in addition to those at molecular and cellular levels. These include climate, rainfall, ocean and air circulation patterns, and extreme weather events, as well as the ecology of the pathogens’ reservoirs and vectors, namely those factors associated with larger-scale mechanisms and the dynamic behavior of ecosystems in which parasite (pathogen) and host relationships are embedded. Still other factors are involved, and must be identified, if a truly holistic framework is to be constructed that incorporates factors related to human and societal mechanisms.
Demographic and social changes, along with associated environmental alterations, and even the efforts to control disease, have contributed to the severity of the problem of EIDs. The use of antimicrobials, pesticides, and biological controls predictably are effecting changes in pathogens, hosts, and ecological systems, and often unwittingly facilitating disease emergence or reemergence. Antibiotic resistant Strepococcus A and E. coli 0:157 are prime examples. Pathogens and their hosts, including humans, reproduce, grow, and adapt in an environmental context, devastatingly exemplified by the avian influenza threat (chickens, ducks, pigs, and humans in close confines). This context is most accurately captured using a holistic or systems perspective, considering sub-systems at different levels of organization—those at lower levels embedded within those at successively higher levels—including social as well as physical, chemical, and biological components.
This view, applied to the extraordinary depth and richness of living systems, spanning the scale of microbial genomes to the regional ecosystems populated by humans and reservoir species, evoked the term biocomplexity. Several investigators, including social scientists, conceived and elaborated on similar themes using different terminology. Ecological and social scientists working on ecosystem and natural resources management challenges refer to “social–ecological systems” or “human and natural systems”. The contraction of social–ecological systems, “socioecological systems,” has been used to describe this same systems perspective, stressing coupled human–natural systems and complexity theory, in the context of health and emerging infectious diseases. The hyphenated or contracted terms share with biocomplexity an emphasis on the interaction of humans and nature as a complex system, and arguably embrace what is fundamentally the same paradigm.