This study is for adults with Cystic Fibrosis (CF) who have a specific lung infection called Pseudomonas aeruginosa. The study will test if a special treatment, called bacteriophage therapy, is safe and helps fight the infection. Bacteriophages are tiny viruses that can kill bacteria. This treatment is given through an IV, which means it goes directly into your bloodstream. The study will take place at up to 20 locations in the USA.

In the first stage, 6 people will try different doses of this treatment. If it seems safe, more people will join to find the best dose. The study will last about 30 days and you will need to attend several visits.

• The study involves several visits within a month.
• Participants will receive the study treatment or a placebo (a harmless substance).
• The study checks for any side effects and the treatment's effectiveness.

To join, you must be at least 18, have CF, and have had the infection before. Pregnant women or those with certain medical issues can't join. If interested, discuss with your doctor.