Empyema is a collection of pus in the cavity between the lung and the membrane that surrounds it (pleural space). The most common cause of empyema is pneumonia caused by a bacterial infection of the lungs. An empyema can form when pneumonia fails to fully respond to treatment in a straightforward way. Some symptoms of empyema are: Excessive sweating, especially night sweats. Fever and chills. General discomfort, uneasiness, or ill feeling (malaise) Shortness of breath.