The greatest postnatal killer of the nineteenth century was puerperal fever. A vicious and usually fatal form of septicaemia, puerperal or childbed fever was known to occur in maternity hospitals far more frequently than at home births, and to spread faster in crowded wards than in those with fewer patients. Its cause was unknown. In this precise statistical analysis of the facts, gathered from several sources across the major cities of Europe, Florence Nightingale (1820-1910) explores the mystery of puerperal fever and its possible causes. She stresses the necessity of good ventilation in hospitals, condemning those with overcrowded wards, and cites instances where the layout of wards has a noticeable correlation with the number of deaths. Published in 1871, just before Pasteur's work on germ theory proved that the problem could be all but eradicated if doctors washed their hands more rigorously, this work remains clear, scholarly and engaging.