Pleural empyema, also known as pyothorax or purulent pleuritis, is empyema (an accumulation of pus) in the pleural cavity that can develop when bacteria invade the pleural space, usually in the context of a pneumonia. It is one of various kinds of pleural effusion. There are three stages: exudative, when there is an increase in pleural fluid with or without the presence of pus; fibrinopurulent, when fibrous septa form localized pus pockets; and the final organizing stage, when there is scarring of the pleura membranes with possible inability of the lung to expand. Simple pleural effusions occur in up to 40% of bacterial pneumonias. They are usually small and resolve with appropriate antibiotic therapy. If however an empyema develops additional intervention is required.