In this study clinical decision making (CDM) is formalized by representing the aetiology and the human body by one directed graph (digraph) and using standard digraph operators (change, add, delete, contract) to model transitions between health and disorder. All nodes of the digraph have the same composite structure <localization, carrier, agent>. For example, an aetiology node is <Ganges river, water, Vibrio cholera>. Paths in the aetiology subdigraph model epidemiological spread. Virulent paths model the entrance into and damage of aetiological agents on the body. Pathogenetic mechanisms make out internal pathways between organs and between cells, and is sharply discriminated from the aetiology. CDM is based on recognizing the difference between healthy and disordered digraphs. The result is a novel and powerful approach to CDM.