Since the writings of Hippocrates, empyema of the thorax has been defined as an accumulation of pus in the pleural space. Some pleural effusions, however, such as those caused by group A streptococci, may be infected without appearing grossly purulent.
Bacteria can reach the pleural space by many routes.
Most often, empyema results from the direct spread of bronchopulmonary infections, including pneumonias, lung abscesses, and bronchiectasis. Of increasing concern is the occurrence of empyema as a complication of thoracotomy or, less often, thoracentesis. Open chest trauma provides another means for the direct introduction of microorganisms. Intra-abdominal infections, especially subphrenic abscesses, can occur through the diaphragm to cause empyemas. Uncommonly, esophageal rupture can cause spread of infection from the mediastinum to the pleural space.