Mesothelioma is a cancerous tumor that comes from the cells of the mesothelium. The mesothelium is a thin protective lining that covers many of the body’s internal organs, including the lungs. The mesothelium has two layers with fluid producing mesothelial cells between these layers. The fluid has a lubricating effect that allows moving organs like the heart and lungs to slide and move easily among other internal body parts with little friction.