The purpose of this work is to present the results obtained after an initial approach to the beliefs and practices concerning the existence of parasites in humans among two Mbya-Guarani communities of the Cuñá Pirú valley, located in the Paranaense Rainforest, northeastern Argentina. The results of this study on the Mbya perspective emerged from two methodological strategies: ethnographic interviews and a series of workshops in the framework of an interdisciplinary research on enteroparasitosis carried out in the region. Morphological criteria ?color and size? are essential for the proper characterization of parasites through the analysis of human feces. Besides these criteria, other parameters are also considered, such as the levels of pathogenicity and virulence, and in turn therapeutic resources. The human-environment duality and different functional criteria characterize natural as well as pathological forms of parasites in the human body. According to the Mbya ethnophysiology,parasites ?called tacho in the Mbya language? are common organisms of the human gastrointestinal tract and integral components of the normal physiological process of digestion. Although their origin is independent of their host, parasites are considered essential for human life. Therefore, the life cycle of parasites depends on that of human beings. However, nutritional deficiencies or external phenomena regarded as harmful for several reasons ?such as inhaling the odor of taboo meat from animals in the monte area, or the effect of startling events on individuals? might suddenly cause ?unrest? among parasites. As a result, the physiological equilibrium of the gastrointestinal system is broken, generating the different symptomatic manifestations of the pathogenic action of parasites. The parasitic reaction may be particularly strong and the disease may range from mild to severe. Besides the aforementioned causes of activation of ?dormant? parasites, other types of parasites may gain entrance into the body from the environment and trigger parasitic diseases. Mbya people may have incorporated this idea into their culture from information released by official healthcare centers. It is in this sense that the corpus of the vernacular theory, which considers parasites as having a functional and endogenous nature, integrates some of the exogenous factors that the scientific field believes are potential causes of parasite infections. According to the Mbya perspective on parasitosis, the non-pathogenic state of parasites may be associated with the perfect equilibrium and harmony between aboriginal communities and the environment ?which depends on living according to ancestral cultural standards?, whereas their pathogenic state is associated with the violation of those standards and the integration of new lifestyles into their culture as a result of their interaction with white people. The transformation of parasites into pathogenic agents, as well as the acceptance of the entrance of exogenous parasites into the human body, resulted from a long historical process of contact and negotiation between vernacular and biomedical perspectives.