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Complexity, simplicity, and epidemiology.

It should be stressed that although many phenomena are complex, 15 the concept of ‘complexity’ is more specific. Complexity is the study of complex adaptive systems. These have been defined as ‘a collection of individual agents with freedom to act in ways that are not always totally predictable, and whose actions are interconnected so that one agent’s actions changes the context for other agents’. 16 Such systems include living cells, the brain, the immune system, the financial markets, ecosystems, and human populations. They are complex in the sense that there are a great many apparently independent agents interacting with each other, but the richness of these interactions allows the system as a whole to undergo self-organization. 1 They are also characterized as involving non-linearity and feedback loops in which small changes can have striking effects that cannot be understood simply by analysing the individual components. 17 The whole is more than the sum of its (reductionist) parts. Such complex systems can exist on a number of different levels from the subatomic through to the individual level, the population level, and beyond. 18 The most striking example of a complex self-organizing system is life itself, not only in terms of individual organisms but also in evolutionary terms—organisms adapt to each other through evolution into a finely tuned ecosystem. Similarly, various populations have evolved traditional ways of life that are now responding to the changes brought by the industrial revolution, colonization, and globalization. 19