The correct identification of diseases that affected Poles in the past centuries is not an easy task. This is due not only to the scarcity of sources but also to the lack of data on clinical symptoms which would be parallel to the data on the patient's medical history obtained by contemporary doctors. It is only the correct identification of illnesses that gives us a fair chance to assess their impact on society and to verify exaggerated views, usually resulting from the fear of death which inevitably accompanied incurable and highly contagious diseases. An interesting region to explore in that respect is Chelmno Land (Kulmerland), whose borders coincide with the old voivodship and bishopric of Chelmno, which after the first and third partition of Poland became part of the Prussian state and later temporarily belonged to the Duchy of Warsaw. The fact that the territory was never divided guarantees the homogeneity of the source material. A crucial element in identifying diseases is a qualitative and quantitative analysis of death records in parish registers, which can bring most interesting results, especially as regards small villages. The record of diseases as causes of death in parish registers in Chelmno Land was started after the territory was incorporated to Prussia. The earliest records date to 1774. The practice was not widespread until 1780; it seems that priests were reluctant to note the cause of death in the registers. The cause of death was noted by the registrar, who was usually the parish priest, on the basis of witnesses' account. With time, the record of death causes was accepted and became more precise, as is evidenced by the growing catalogue of the illnesses that appear in the records. In the case of common and well-known diseases, such a smallpox, the identification can be assumed trustworthy; with new illnesses, however, such as cholera in 1831, the first cases may not have been diagnosed properly and may have been called diarrhoea. The description of the cause of death usually included general symptoms; initial causes were mentioned rarely, while secondary and direct causes were neglected. The record was certainly influenced by individual priests, who, having collected information on symptoms from witnesses, identified illnesses according to their knowledge. Such practice is suggested by fixed lists of illnesses detectable in the records made by particular priests, which partly change with the change of the registrar. It is not particularly difficult to link Latin and German terms with Polish names of diseases. Yet, the recorded popular names are often so general that a precise identification of an illness is problematic. Parish registers do not offer a survey of all illnesses, being limited to those that were common causes of death.