Causes Empyema is usually caused by an infection that spreads directly from the lung. It leads to a buildup of pus in the pleural space. There can be 2 cups (1/2 liter) or more of infected fluid. This fluid puts pressure on the lungs. Risk factors include: Bacterial pneumonia Tuberculosis Chest surgery Lung abscess Trauma or injury to the chest In rare cases, empyema can occur after thoracentesis. This is a procedure in which a needle is inserted through the chest wall to remove fluid in the pleural space for medical diagnosis or treatment