Aztreonam-Avibactam in Immunocompromised Hosts

The goal of this observational study is to learn about the effectiveness and safety of aztreonam-avibactam (ATM-AVI), an antibiotic used to treat infections caused by highly resistant bacteria, in adults with weakened immune systems. The main questions this study aims to answer are: * How well does ATM-AVI work in treating these infections? * How many participants recover, and how many die or develop another infection caused by the same bacteria? * What side effects or medical problems occur during or after treatment? Researchers will also compare participants treated with ATM-AVI with a group of participants previously treated with ceftazidime-avibactam plus aztreonam (CZA-ATM). This comparison will help researchers understand differences in treatment outcomes. The study will include adults with weakened immune systems, such as people with cancer, organ transplants, or those receiving medicines that suppress the immune system. Participants will: * Receive antibiotic treatment chosen by their treating physicians as part of routine medical care. * Have their medical records reviewed to collect information about their infections, treatments, and outcomes. * Be followed for up to 60 days after starting treatment to assess recovery, survival, side effects, and recurrence of infection. * Researchers will not assign treatments or change participants\' medical care.