Pneumonia: Overview and Epidemiology

Pneumonia is the infection of the distal lower respiratory tract, principally the alveolar space, including the small bronchi and bronchioles. Pneumonia results from the proliferation of microorganisms at these sites in combination with the host response to the presence of microorganisms. A variety of pneumonia syndromes can be defined based on microbial etiology, underlying host defenses, clinical presentation, and site of acquisition. Because of the non-specific nature of the signs and symptoms, the diagnosis of pneumonia rests disproportionately on the presence of an infiltrate on chest radiograph. The frequent lack of a microbiologic diagnosis and the non-specificity of signs, symptoms, and radiographic infiltrates leads to frequent consideration of pneumonia in the differential diagnosis of many other pulmonary diseases. The etiologic spectrum of all types of pneumonia always includes the common community-acquired pathogens, with the spectrum broadening in response to either increasing immunocompromise or greater exposure to broad-spectrum antibiotics. While new or previously unrecognized pathogens can play a role in management decisions, the major issue in pneumonia is the appearance of antibiotic resistant strains of known common bacterial pathogens. Lack of even minimally effective agents is more the issue for nonbacterial pneumonias. Immunomodulation, in the form of less immunosuppressive cancer and transplant chemotherapy, immunization, or specific immunomodulatory drugs holds some hope for improving pneumonia outcome in the future.