Lung abscess is a chronic or subacute lung infection initiated by the aspiration of contaminated oropharyngeal secretions. The result is an indolent, necrotizing infection in a segmental distribution limited by the pleura. Except for infections with unusual organisms such as Actinomyces, the process does not cross interlobar fissures, and pleural effusion is uncommon. The resultant cavity is usually solitary, with a thick, fibrous reaction at its periphery. So defined, lung abscess is almost always associated with anaerobic bacteria, although microaerophilic and aerobic bacteria are frequently present as well.

In contrast, necrotizing pneumonia is an acute, often fulminant, infection characterized by irregular destruction of alveolar walls and therefore multiple cavities. This infection spreads rapidly through lung tissue, frequently crossing interlobar fissures, and is often associated with pleural effusion and empyema. The duration of illness before recognition is usually only a few days. Causative organisms include Staphylococcus aureus, Streptococcus pyogenes, Klebsiella pneumoniae, Pseudomonas aeruginosa, and, less commonly, other gram-negative bacilli, Legionella species, Nocardia species, and fungi.

The focus of this discussion will be the diagnosis and therapy of anaerobic lung abscess. Diagnosis can usually be made from the clinical presentation and chest radiograph findings. Many patients have conditions such as seizure disorders, neuromuscular diseases, alcoholism, or other causes for impaired consciousness that predispose them to aspiration of oropharyngeal secretions.