From antiquity, parasites, and especially worms, were thought to be responsible for human suffering and disease. However, in the seventeenth and eighteenth centuries, worms became the subject of extensive scientific investigations and began to be implicated in a much wider array of diseases. The advent of widespread use of the compound microscope for scientific investigation in the mid-seventeenth century contributed to a flourishing of research into parasitic organisms, particularly worms, and their role in disease. Although historians of medicine have written about the history of parasitology, almost all of these studies begin with the formal establishment of parasitology as a scientific discipline in the latter half of the nineteenth century. The preceding two centuries of parasitological research, however, remain relatively unexamined. In this project, I argue that parasites, especially worms, were important explanatory mechanisms for a wide range of diseases during the early modern period. Thus, the neglect of early modern parasitology by historians of medicine means that we have missed a crucial aspect of medical theory in this period. This project contributes to our understanding of early modern ideas about disease and disease causation by challenging existing historiographical categories.