What is the difference between pneumonia and pneumothorax?

Pneumonia is an infection that inflames the air sacs in one or both lungs, which may fill with fluid or pus. The term combines pneumo meaning lung and ia indicating a condition. Pneumothorax is a collapsed lung caused by air leaking into the space between the lung and chest wall. It combines pneumo meaning air and thorax meaning chest. Despite sharing the prefix pneumo, the conditions are completely different: pneumonia is an infectious and inflammatory process, while pneumothorax is a mechanical problem where air in the wrong space compresses the lung.