Infection, tumor, CHF. A pleural effusion is just fluid in the space between lung and rib cage. Normally this 'potential space' is empty with a scant amount of fluid to allow the lung to expand/and slide inside the rib cage. Inflammation/infection/tumor may cause fluid to build up in the pleural space causing a pleural effusion. The fluid can be 'tapped' for analysis. Another cause is related to heart failure (chf).