Epidemics, where a whole population suffers from a sudden and usually fatal illness, were difficult to understand in medieval Christian Europe. On the one hand, to believers, epidemics were clearly the action of God, warning or punishing society. On the other, the dominant theoretical medical teaching, derived from Galen, dealt with individuals, not people in the mass. But suffering societies were not passive: they frequently sought out scapegoats such as lepers, Jews or simply foreigners, believing them to have deliberately poisoned the water supply. Jon Arrizabalaga explores these issues in fourteenth-century Southern Europe, focussing on the so-called ‘Black Death’ of 1348–49. He shows that the fear caused by epidemics in turn led to the belief that the poison of epidemics could not only be spread deliberately by evil persons, but also be man-made, often in the form of powders.