In many countries across the world at different times, a diet made up exclusively of maize led to the development of B3 avitaminosis, or pellagra. Caused by extremely limited nutrition, B3 avitaminosis is a deficiency disease due to insufficient intake of niacin and tryptophan. From the late eighteenth century up to the time of the First World War, pellagra was endemic in Northern Italy, particularly in the Veneto. The “sickness of the poor” and the turmoil it caused affected a single social class whose diet consisted entirely of cornmeal polenta: farm workers, especially day labourers, a particularly underprivileged occupational category. This multidisciplinary analysis, based on various types of documentary sources, retraces the epidemiological, social, political and demographic mechanisms that led to the spread of pellagra, primarily among women farm workers of reproductive age in the Veneto and Lombardy regions. Observation of the demographic impact of the disease on the female peasant population leads to a discussion of possible effects on the birth and fertility rates of the populations of these two regions in the late nineteenth century.