This study tests bacteriophage therapy for people with Cystic Fibrosis (CF), a lung disease. A bacteriophage is a virus that attacks bacteria. The study focuses on people who have a lung infection caused by the bacteria Pseudomonas aeruginosa. Participants will get one dose of bacteriophage therapy through an IV, which means medicine is given through a needle into a vein. The study checks safety and how well the treatment works. It will take place in the US at up to 20 sites.

In Stage 1, the therapy will be tested on six people. If no serious side effects appear, the study will move to Stage 2, including more people. Participants will be randomly placed in groups receiving either the bacteriophage or a placebo (a harmless substance used to compare results).

• Participants must be 18 or older with CF and a confirmed infection from the bacteria.
• The study lasts about a month with several visits.
• Participants must not change chronic medications during the study and must not be pregnant or planning to be.