Malignant mesothelioma is cancer that occurs in the thin tissue layer that covers most internal organs (mesothelium). Mesothelioma is a form of aggressive and deadly cancer. There are treatments for mesothelioma, but for many people with mesothelioma, healing is impossible. Doctors divide mesothelioma into different types, depending on which part of the mesothelium is affected. Mesothelioma usually affects the tissue that surrounds the lungs (the pleura). This type is called pleural mesothelioma. Other, more rare types of mesothelioma affect the abdominal tissue (peritoneal mesothelioma) around the heart and around the testicles.