All the lungs were analyzed by microscopy, even when the medical records indicated the patient's diagnosis. The lungs were fixed in 10% formalin prepared in 0.9% saline for at least four weeks. We studied a minimum of five sections per lung for a total of ten sections per person, regardless of the presence or absence of morphologically demonstrable lesions. The paraffin-embedded tissue sections were assessed following hematoxylin and eosin staining. To document the presence and distribution of the wide spectrum of infectious agents and neoplasms to which this population was susceptible, we prepared a variety of special stains (periodic acid-Schiff staining, immunohistochemical analysis, fluorescence, Ziehl-Neelsen acid-fast staining, Gram acid-fast staining, Mucicarmine acid-fast staining, and Gomori's methenamine silver acid-fast staining) for selected tissue sections. Bacterial bronchopneumonia (BBP) was defined as the presence of cell consolidation with polymorphonuclear leukocyte accumulation in bronchioles and adjacent alveoli. For the diagnoses of cytomegalovirus and fungal pneumonia, histological evidence of lung involvement was required, with or without tissue culture.