Nocardia bacteria are common saprophytes in the soil and water. This bacteria does not commonly colonize the upper respiratory tract. Therefore, when the organism is isolated it should be considered a pathogen. Two species of Nocardia account for most of the disease seen in humans. Nocardiosis also has been described in patients with chronic obstructive lung disease, diabetes, systemic lupus, and pulmonary alveolar proteinosis.
Nocardia infection shows both suppurative and granulomatous features.
Pulmonary lesions are characterized principally by necrotizing abscess that are not well encapsulated and therefore spread easily. Granuloma formation may occur but requires some degree of cell mediated immune competence. The typical radiologic manifestations are nodules and areas of consolidation, with or without cavitation, that frequently abut the pleural space. Spread to the central nervous system is common. Chest wall invasion may occur but is uncommon.