Immunocompromised patients develop infections resulting from a wide range of organisms. The most commonly encountered type of infection is bacterial in origin. Many of the infections are community-acquired pneumonias in which most of the infections are caused by organisms that typically produce disease in the healthy person. Hospital-acquired pneumonias are particularly serious, being caused by the highly virulent gram-negative bacilli and Staphylococcus aureus. Immunocompromised patients frequently have indwelling intravascular catheters. These catheters may become infected and seed the lung with septic emboli, producing a hematogenous pneumonia. Underlying conditions and therapy increase the risk for aspiration in the immunocompromised patient. These aspirations can result in the development of an aspiration pneumonia and lung abscess formation. The majority of pneumonias resulting from Legionella and Nocardia occur in immunocompromised patients.