From 'two medicines' to 'One Health' and beyond.

We first review historic and conceptual background to integrative thinking in medicine. Lacking a general theory of 'One Health', we provide an operational definition of 'One Health' and its leverage as: any added value in terms of human and animal health, financial savings or environmental benefit from closer cooperation of human and animal health sectors at all levels of organisation. Examples of such added value of 'One Health' are given from the fields of health systems, nutrition and zoonoses control in Africa and Asia. 'One Health' must become main-stream rather than a new discipline or new association; it should just become normal that practitioners and professionals in the health, animal and environment sectors work together as closely as possible. Current and future challenges in financing clean energy, migration flows, food security and global trade further warrant rethinking of human and animal health services. A conceptual outlook relates health as an outcome of human-environment systems called 'health in social-ecological systems'. The paper ends with an outlook on the operationalisation of 'One Health' and its future potential, specifically also in industrialised countries.