As shown in the diagram, there is a potential gap between the lungs and the chest wall, which, in a healthy state, contains a little fluid that serves as a lubricant. When sick, the fluid increases and is called pleural effusion, or pleural fluid. It can be unilateral or bilateral. Common causes: (1) tumor —- cancerous pleural effusion (2) tuberculosis —- tuberculous pleural effusion (3) common bacterial infection —- abscess chest (4) liver, kidney, heart disease and malnutrition can cause pleural effusion. Treatment: The methods vary according to the cause.