Neither the sun nor death can be looked at with a steady eye. —La Rochefoucauld
The political scientists Harold and Margaret Sprout, pioneers in the application of ecological principles to the domain of political science, noted that when complex connections exist between elements "any substantial change in one sector of the milieu is nearly certain to produce significant, often unsettling, sometimes utterly disruptive consequences in other sectors."42 As such, tracing processes though case studies permits a holistic examination of the system wherein "the properties of the whole . . . can only be discovered by studying the whole."43 The political scientist Robert Jervis concurs, arguing that systems are typically highly complex and contingent.44 Jervis goes on to define the properties of a system:
We are dealing with a system when (a) the set of units or elements is interconnected so that changes in some elements or their relations produce changes in other parts of the system, and (b) the entire system exhibits properties and behaviors that are different from those of the parts. . . . The result is that systems often display nonlinear relationships, outcomes cannot be understood by adding together the units of their relations, and many of the results of actions are unintended. Complexities can appear even in what would appear to be simple and deterministic situations.45
Thus, natural systems exhibit profound complexity. Small changes that are gradually introduced over time may induce temporally distal non-linearities, whereupon the system suddenly shifts to a new equilibrium.46 Thus, human attempts to tame nature typically result in unforeseen negative outcomes. One notable example of such an outcome was the use of DDT to wipe out mosquitoes in order to diminish the transmission of such arthropod-borne diseases (e.g., malaria). Regrettably, the use of DDT resulted in widespread destruction of avian populations and generated evolutionary pressures that eventually created resistant populations of the vector.
The chains of connections among disparate elements (or variables) within a system may be exceedingly complex, often involving feedback loops and contingencies. The relationship among a pathogen, a vector, and a host is complex, and the debilitation or destruction of the human host will then generate externalities throughout a society that reverberate across various domains (e.g., demographic, psychological, economic, political). Indeed, outcomes in one domain may affect outcomes in other domains, creating feedback loops that perpetuate constant evolution of the system.47 In addition, many bio-political systems may exhibit lag effects, wherein outcomes are delayed, are indirect, and intensify over the longer term.48
Furthermore, when examining the effects of disease on political structures, we may find that iterated indirect effects trump direct effects in their influence on systemic outcomes. Preliminary evidence suggests that the indirect effects of health on state capacity are attenuated and are likely to increase over the longer term.49
Aristotle acknowledged principles of non-linearity and randomness when he commented that "things do, in a way, occur by chance."50 Variables within complex systems often interact to generate outcomes not predicted by linear models. Non-linear functions are often observed in natural systems, the obvious example being booms and subsequent crashes in the population of an animal species. Epidemiological curves also illustrate principles of non-linearity, wherein variables combine to generate exponential growth in the rates of infection, then plateau, and then crash as acquired immunity in the infected host population reaches a self-sustaining critical point.51
In his description of the SARS epidemic that struck Toronto in 2003, Justice Archie Campbell, Chief Commissioner of Canada's SARS Commission, described the contagion as a "perfect storm" of diverse factors.52 This analogy is appropriate: epidemics often exhibit "emergent properties," in that the manifestation of contagion possesses characteristics that are greater and perhaps quite different than its constituent parts and that might be quite unexpected. As the sociologist Emile Durkheim commented, "whenever certain elements combine and thereby produce, by the fact of their new combination, new phenomena, it is plain that these new phenomena reside not in the original elements but in the totality formed by their union [or interaction]."53 Thus, complex systems may exhibit properties that are attributable not to their discrete components, but rather to the macro-level interaction of those components, and thus
(under conditions of strong emergence) the whole may be both greater than and different from the sum of its parts.54
Emerging pathogens, and their manifestations in epidemic or pandemic form, often exhibit "emergent properties" resulting from the interaction of variables in complex and interdependent global systems. The collectivity may not only be a function of the combination of direct effects of the discrete variables within a system; it may also be a function of unanticipated side effects of these variables. For example, population growth and increasing population density generates unanticipated side effects that permit the zoonotic transmission of disease into, and expansion throughout, the human ecology. The exceptionally dense and large aggregate populations of "mega-cities" can act as population pools that support the endogenous (and continuous) transmission of certain pathogens. The classic example of this is measles, which can maintain steady transmission rates only in cities with populations of at least 250,000.55 Therefore, the rise of huge and concentrated new urban population centers, coupled with environmental degradation and rapid migration, may permit the endogenization of new pathogenic zoonoses (e.g., SARS) within the human ecology.
The concept of chaos is also applicable to evolutionary changes in the genetic structures of pathogenic agents. As the virologist Joshua Lederberg pointed out, the genetic structures of pathogens are highly mutable, and changes in traits of transmissibility and lethality are often governed by chance mutation.56 The classic example is the influenza virus, whose genetic structure is constantly shifting and changing. The evolutionary trajectory of influenza is decidedly non-linear in nature and, consequently, highly unpredictable. As was demonstrated by the H5N1 variant of the virus, it could rapidly evolve into a lethal pandemic (along the lines of the 1918 "Spanish Flu") or it could simply mutate into a relatively benign and non-pathogenic variant (as happened with Swine Flu in 1976).
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