This book explores the nature of COVID-19 through posing four questions: What could be said to be the cause of COVID-19? Who is likely to be infected? Where is infection likely to take place? When is infection likely to occur? It will demonstrate that these four W questions are interconnected, that they are the very ones which pre-occupy biomedicine today, whether in the form of clinical medicine or epidemiology. Epidemiology is often portrayed as “sub-standard”, while clinical medicine is put on a pedestal, marked as “superior” and “methodologically impeccable/privileged”. This book challenges this standard assessment. It argues that the causal model underpinning epidemiology is more adequate to account for medical data and evidence, across the board. Epidemiological Thinking is Medical Thinking, which this book calls Ecosystem Thinking and, through it, explores the values underpinning Medical Thinking, including geopolitical values.