Mesothelioma is a rare form of cancer in which malignant (cancerous) cells are found in the mesothelium, a protective sac that covers most of the body’s internal organs. Most people who develop mesothelioma have worked on jobs where they inhaled asbestos particles. Virtually all cases of malignant mesotheliomaare are attributable to asbestos exposure, which is why mesothelioma is often referred to as "asbestos cancer."