This is the second in a series of four articles

Biological and social systems are inherently complex, so it is hardly surprising that few if any human illnesses can be said to have a single “cause” or “cure.”1 This article applies the principles introduced in the introductory article in this series2 to three specific clinical areas: the control of blood glucose levels in diabetes, the management of diagnostic uncertainty, and health promotion.

A complex adaptive system is a collection of individual agents with freedom to act in ways that are not always totally predictable, and whose actions are interconnected so that the action of one part changes the context for other agents.2 In relation to human health and illness there are several levels of such systems.

• The human body is composed of multiple interacting and self regulating physiological systems including biochemical and neuroendocrine feedback loops

• The behaviour of any individual is determined partly by an internal set of rules based on past experience and partly by unique and adaptive responses to new stimuli from the environment

• The web of relationships in which individuals exist contains many varied and powerful determinants of their beliefs, expectations, and behaviour

• Individuals and their immediate social relationships are further embedded within wider social, political, and cultural systems which can influence outcomes in entirely novel and unpredictable ways

• All these interacting systems are dynamic and fluid

• A small change in one part of this web of interacting systems may lead to a much larger change in another part through amplification effects.

For all these reasons neither illness nor human behaviour is predictable and neither can safely be “modelled” in a simple cause and effect system.3 The human body is not a machine and its malfunctioning cannot be adequately analysed by breaking the …