What is mesothelioma? Mesothelioma is a cancer that, although rare, still claims thousands of lives every year. It affects the mesothelium, a layer of mesothelial cells that lines the specific parts of your body and most of the internal organs. There are four types of mesothelioma, with the most common form affecting the pleura, the thin membrane lining the lungs (pleural). This is because the cause of mesothelioma, asbestos fibers, usually enters the body when inhaled. The lesser common types are peritoneal, which affects the abdomen, pericardial, which affects the heart, and testicular (the rarest type). All of these are caused by inhaling or digesting asbestos fibers.