PHAGEFORCE is a prospective, observational registry study. The University Hospitals Leuven has approved the application of phage therapy as standard-of-care only in patients for whom no curative treatment alternatives (antibiotic and/or surgical) are available ('last-resort cases'). A multidisciplinary phage task force, referred to as the Coordination group for Bacteriophage therapy Leuven (CBL) was set up. The CBL screens patients with difficult-to-treat infections, evaluates who could benefit from phage therapy and sets up the treatment protocol. With this study, the CBL aims to gain insight in the safety and efficacy of phage therapy by integrating and optimizing phage therapy in five distinct medical disciplines (with distinct routes of administration), facilitating long-term follow-up of patients. Furthermore, this study will gain insight in the biodistribution and exact mechanisms of action of phage therapy and thus be able to provide standardized guidelines for each patient population and route of administration.