[Complexity of disease; a modern view in times of ageing populations and multimorbidity]

Complexity of patient care is rapidly increasing as a consequence of rising numbers of patients with complex multimorbidity. Not just the patient as a whole, but also the networks of organs, tissues and cells are forming a complex adaptive system (CAS). A CAS is defined as a network of several components ('agents') with lots of mutual feedback loops between which there are circular causalities; the predictability of a CAS is limited by definition. However, current guidelines and evidence-based medicine assume that diseases and the medical interventions to address them are predictable. Physicians' brains are complex neural networks that are much better at dealing with complex situations than guidelines. In the near future, physicians will also get help from advanced computer simulation models that make better diagnostic analyses on the basis of detailed phenotyping and are more accurate when predicting possible courses of disease and treatment outcomes.