Infections caused by viruses, pneumonia, or heart failure are examples of pleural effusion causes that can be efficiently treated or managed. Other causes may not be as easily resolved but still require prompt attention including cancer and liver disease. Pulmonary effusions are often a sign of a serious problem with no simple solution. They can occur for many reasons and affect different areas of the lungs. Knowing the cause will help determine the best course of action. Pleural effusions are fluid collections between the lining of the chest cavity and the lung tissue. The two main types are transudates and exudates. With transudates, blood vessels leak into the space between cells causing fluid to flow out of the blood and into the lung. With exudates, there is damage to the alveoli (air sacs in the lungs) causing protein-rich fluid to leak into the space around them. The three main factors in determining the cause of pleural effusion are history taking, physical examination, and laboratory tests. Imaging studies such as chest x-rays and CT scans may be used to identify additional problems not detected by these first methods. A biopsy may be required if doctors cannot find the cause after an extensive workup.