Opportunistic infections are a major cause of morbidity and mortality in severely immunocompromised patients, such as those receiving chemotherapy or biological therapies, patients with haematological malignancy, aplastic anaemia or HIV infection, and recipients of solid-organ or stem cell transplants. The type and degree of the immune defect dictate the profile of potential opportunistic pathogens; T-cell-mediated defects increase the risk of viral (cytomegalovirus, respiratory viruses) and Pneumocystis jirovecii infections, whereas neutrophil defects are associated with bacterial pneumonia and invasive aspergillosis. However, patients often have combinations of immune defects, and a wide range of other opportunistic infections can cause pneumonia. Importantly, conventional non-opportunistic pathogens are frequently encountered in immunocompromised hosts and should not be overlooked. The radiological pattern of disease (best assessed by computed tomography) and speed of onset help to identify the likely pathogen(s); radiological imaging can subsequently be supported by targeted investigation including the early use of bronchoscopy in selected patients. Rapid and expert clinical assessment can identify the most likely pathogens, which can then be treated aggressively, providing the best opportunity for a positive clinical outcome
