Pneumonia, most commonly a bacterial infection of the alveolar lung, afflicts millions in the United States yearly, remaining a leading cause of morbidity and mortality. Pneumococcus (Streptococcus pneumoniae) is the classic bacterial etiology, though incidence from atypical and opportunistic agents, particularly if pneumonia is acquired in health care settings, is increasing. Staphylococcus aureus, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Haemophilus influenzae are additional causative bacterial agents. Legionella pneumophila, Mycoplasma pneumoniae, Chlamydia pneumoniae, and a spectrum of respiratory viruses account for the bulk of atypical pneumonias. Anaerobes are less frequently encountered, but must be highly suspected in circumstances of likely aspiration. Risk factors for pneumonia are multiple, and include diseases of the respiratory tract (eg, chronic obstructive pulmonary disease or COPD) and immune system (eg, cancer, AIDS), as well as chronic conditions associated with aspiration, bacteremia, and debilitation.