Pleural space infections with + Gram stain/culture OR parapneumonic effusions without pleural fluid sampling
Stages
Exudative - Free-flowing pleural effusion amenable to chest tube drainage; may only last <48hr
Fibrinopurulent - Loculations develop making resolution with single chest tube drainage unlikely
Organizational - Takes several weeks to develop; "pleural peel" restricts lung expansion
Pneumonia (most common)
Complications of chest or abdominal trauma
Esophageal perforation
Extension from lung abscess
Osteomyelitis or other near pleural infections
Hemothorax, chylothorax, or hydrothorax that becomes infected
Empyema
Pneumonia
Atypical pneumonia
Sarcoidosis
Bronchiolitis 
Tuberculosis
Acute bronchitis
Pleural Effusion