What is Mesothelioma? The National Cancer Institute states that: "Malignant mesothelioma, a rare form of cancer, is a disease in which cancer (malignant) cells are found in the sac lining the chest (the pleura), the lining of the abdominal cavity (the peritoneum) or the lining around the heart (the pericardium). How do you get Mesothelioma? Most people with malignant mesothelioma have worked on jobs where they breathed asbestos. Others have been exposed to asbestos in a household environment, often without knowing it.