Of the three types of mesothelioma, a cancer impacting on the mesothelium, malignant pleural mesothelioma is the most widespread. The cancer malignancy is rare and only fewer than 3000 mesothelioma cases are documented yearly in the United States. The pleural type, which we are looking at here, makes up about three-fourths of it. The cancer affects the pleura, a protecting sac that guards the lungs. This sac, which shores up the lungs and shields the lung cavity, is built of two layers. The inner lining or the visceral lining sheaths the lungs while the outer one guards the chest cavity and diaphragm. Pleural mesothelioma happens initially in any one of these layers and may spread into the other one after a period of time. This cancerous growth is caused by asbestos fibers ensnared in the open spaces between mesothelium cells.