Damaged airways. Bronchiectasis: structurally damaged airways unable to effectively move mucous. Air contains germs that are trapped but not killed by the inner mucous layer of the breathing tubes. Cilia on underlying cells beat like brooms sweeping mucous to the throat where it's swallowed. Stomach acid kills the germs. Without this mucociliary escalator germs multiply in the airway causing bronchitis & pneumonia.
A dead-end bronchus. Oxygen exchange takes place in the small air sacs in the lung. When the air sacs are destroyed, the airways (bronchi) end up in a dead end. Consequently the normal flow of mucus is interrupted and a stagnant space, like a still pond of water, is more susceptible to get infected. Bronchiectasis is commonly found in people with cystic fibrosis or allergic bronchopulmonary aspergillosis (abpa)