Although bacteria are the most common group of microorganisms that cause pneumonia, the spectrum of organisms known to cause respiratory infections is broad and constantly increasing. Imaging plays an important role in the diagnosis and treatment of patients with respiratory infections. Chest radiography helps in the diagnosis by depicting lung opacities and in the evaluation of the progress of disease and response to treatment. Thin-section computed tomography is particularly important in early diagnosis in patients with a high clinical suspicion of disease and a negative chest radiograph. In the absence of clinical information, radiologists cannot reliably distinguish between pneumonia and other pulmonary processes. The radiographic manifestations of a given organism may be variable depending on the immunological status of the patient as well as by pre- or coexisting lung disease or significant comorbidities. Patients with impaired immune function are susceptible to infections by a wide range of organisms. Mildly immunocompromised patients with chronic debilitating illness, diabetes, malnutrition, alcoholism, advanced age, prolonged corticosteroid administration, and chronic pulmonary diseases are prone to develop a semi-invasive or chronic necrotizing aspergillosis. An extensive number of non-infectious processes including drug-induced pulmonary disease, acute eosinophilic pneumonia, organizing pneumonia, and pulmonary vasculitis may mimic pulmonary infection.