Empyema is also called pyothorax or purulent pleuritis. It’s a condition in which pus gathers in the area between the lungs and the inner surface of the chest wall. This area is known as the pleural space. Pus is a fluid that’s filled with immune cells, dead cells, and bacteria. Pus in the pleural space can’t be coughed out. Instead, it needs to be drained by a needle or surgery.