Bronchiectasis is characterized by permanent, abnormal dilation of bronchi due to a vicious cycle of infection, inflammation, and structural airway damage. Cole's vicious cycle hypothesis describes how impaired mucociliary clearance (from ciliary dysfunction, mucus hyperviscosity, or airway damage) allows bacterial colonization, which triggers neutrophilic inflammation and release of proteases (neutrophil elastase, matrix metalloproteinases) that damage airway walls, causing further dilation and impaired clearance. Common predisposing conditions include post-infectious damage (childhood pneumonia, pertussis, measles, TB), cystic fibrosis (CFTR mutation), primary ciliary dyskinesia (dynein arm defects), immunodeficiency (common variable immunodeficiency — CVID), allergic bronchopulmonary aspergillosis (ABPA), and autoimmune diseases (RA, inflammatory bowel disease). The most common colonizing organisms are Haemophilus influenzae, Pseudomonas aeruginosa (associated with worse prognosis), and Moraxella catarrhalis.