COPD is associated with increased bacterial colonisation in the large airways and impaired macrophage phagocytosis and production of cytokines. Inhaled
corticosteroids reduce bacterial adherence and, therefore, might favour displacement of bacteria to the distal airways and might also increase bacterial loads and
change the respiratory microbiome. Inhaled corticosteroids might also further impair macrophage function. An additional trigger such as a virus infection might
produce further acute impairment of host immune functions, resulting in increased bacterial growth and the clinical syndrome of pneumonia.