Viral and Pneumocystis carinii infections of the lung in the immunocompromised host

Viruses and Pneumocystis carinii are significant causes of pneumonia in immunocompromised patients, particularly patients with impaired cell-mediated immunity. They are often simultaneously considered in the differential diagnosis of diffuse pneumonitis in these patients and, because radiographic appearances and the periods of vulnerability to these infections may overlap, may be difficult to differentiate. This article will correlate radiographic findings to evolving histopathologic changes in select specific infections, as they affect three different immune impaired populations: (a) acquired immunodeficiency syndrome, (b) immunosuppression secondary to therapy for underlying malignancy or with solid organ transplantation, and (c) immune impairment in the bone marrow transplant patient. Appreciation of the specific clinical setting in which to consider these infections will be emphasized.