The Israelites believed all ill health was caused by the Almighty's displeasure, and the idea persisted in some parts of Christian society for more than two millennia. Today, we understand that the good health, and indeed the survival, of any mammal depends on three things: resistance to infectious disease, an appropriate diet, and avoidance of poisonous substances. Our defences against microbes are complicated and powerful, and although they may protect us, they can also create symptoms of disease. We can define our dietary requirements as never before and can recognise that deficiencies of these nutrients have blighted human societies throughout history, although we still have some way to go to ensure the optimum micronutrient intake. The human body is vulnerable to poisons in plant foods and to environmental pollution, in spite of the extraordinary powers we have to detoxify natural and man-made poisons. This chapter tells the story of how, in the twentieth century, we came to understand some of the requirements for a healthy life.