Causes of Empyema
The lungs and inside of the chest cavity are lined with a smooth layer called the pleura. These layers are almost in contact, but separated by a thin space – the pleural space – filled with a small amount of lubricant called pleural fluid.

The pleural fluid can sometimes build up and become infected, so that a collection of pus forms. This can thicken and cause areas of the pleura to stick together, creating pockets of pus.

Empyema can worsen to become many more pockets of pus, with thick deposits coating the outer layer of the lungs. These deposits prevent the lungs expanding properly.

Pneumonia and other possible causes
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. Other possible causes are:
Bronchiectasis, a long-term condition where the airways of the lungs become abnormally widened, leading to a build-up of mucus that can make the lungs more vulnerable to infection.

A blood clot or other blockage preventing blood flow to the lungs, causing some of the lung tissue to die. This is known as a pulmonary infarction.

Surgery to the chest (empyema is a rare complication).

An endoscopy (empyema is a rare complication).

A serious injury to the chest.

An infection elsewhere in the body that has spread via the bloodstream.

An infection caused by inhaled food, if you have swallowing problems.
Tuberculosis