None of the recent studies stimulated by debates on the causes of the ‘demographic transition’ since the mid-nineteenth century has yet explained the anomaly that, while all other indices of mortality declined, rates of infant mortality were static or even rising in the last decades of the century. This case study of Preston shows that an explosive growth in the population of horses in expanding towns was probably responsible for sustaining, or even enhancing, levels of infant mortality due to enteric diseases spread by flies which bred in horse manure.